An attempt was made to estimate the error in the determination of the vitamin A value (activity) when the cis- and trans-isomers are not separated and quantified individually. In 10 sample lots of fresh tomatoes, cis-isomers were not detected. However, in 52 samples of tomato products, varying amounts of cis-isomers of β-carotene were found. Utilising the currently employed calculation procedure, overestimations of 8-13% in juice, 10-66% in puree and paste, 15-26% in ketchup were observed when isomer separation was not undertaken. When column loss was taken into consideration, these errors decreased to 4-8%, 6-62% and 11-21%, respectively. Using the biopotencies obtained by Sweeney and Marsh (1970), aside from column loss, the overestimations were 4-8%, 3-52% and 7-15%, respectively. Thus, the error can be considerable, but the magnitude depends on how the calculation is carried out, demonstrating the necessity of establishing the true biopotencies, aside from obtaining more efficient chromatographic separation and accurate quantitation. © 1992.454297302Bickoff, Atkins, Bailey, Sttit, Stereo-isomeric analysis of β-carotene (1949) J. Assoc. Off. Anal. Chem., 32, pp. 766-774Bushway, Separation of carotenoids in fruits and vegetables by high performance liquid chromatography (1985) J. Liq. Chromatogr., 8, pp. 1527-1547Chandler, Schwartz, HPLC separation of cis-trans-isomers in fresh and processed fruits and vegetables (1987) Journal of Food Science, 52, pp. 669-672Davies, Carotenoids (1976) Chemistry and Biochemistry of Plant Pigments, 2, pp. 38-65. , 2nd edn, T.W. Goodwin, Academic Press, LondonDeuel, Jr, Johnson, Sumner, Polgár, Zechmeister, Stereochemical configuration and provitamin A activity I. All-trans-β-carotene and neo-β-carotene U (1944) Arch. Biochem., 5, pp. 107-114Deuel, Jr, Johnston, Merserve, Polgár, Zechmeister, Stereochemical configuration and provitamin A activity IV. Neo-β-carotene B and neo-β-carotene U (1945) Arch. Biochem., 7, pp. 247-255Heinonen, Ollilainers, Linkola, Vaso, Koivistoinen, Carotenoids in Finnish foods: vegetables, fruits and berries (1989) J. Agric. Food Chem., 37, pp. 655-659Khachik, Beecher, Separation and identification of carotenoids and carotenol fatty acid esters in some squash products by liquid chromatography I. Quantitation of carotenoids and related esters by HPLC (1988) J. Agric. Food Chem., 36, pp. 929-937Khachik, Beecher, Whitaker, Separation, identification and quantification of the major carotenoids and chlorophyll constituents in extracts of several green vegetables by liquid chromatography (1986) J. Agric. Food Chem., 34, pp. 603-616Khachik, Beecher, Lusby, Separation identification and quantification of the major carotenoids in extracts of apricots peaches cantaloupe and pink grapefruit by liquid chromatography (1989) Journal of Agricultural and Food Chemistry, 37, pp. 1467-1473Kimura, Rodriguez-Amaya, Godoy, Assessment of the saponification step in the quantitative determination of carotenoids and provitamin A (1990) Food Chem., 35, pp. 187-195Pettersson, Jonsson, Separation of cis-trans isomers of alpha and beta-carotene by adsorption HPLC and identification with diode array detection (1990) J. Micronutr. Anal., 8, pp. 23-41Quackenbush, Reverse phase HPLC separation of cis- and trans-carotenoids and its application to β-carotene in food materials (1987) J. Liq. Chromatogr., 10, pp. 643-653Quackenbush, Smallidge, Nonaqueous reverse phase liquid chromatographic system for separation and quantification of provitamins A (1986) J. Assoc. Off. Anal. Chem., 69, pp. 767-772Rodriguez-Amaya, Critical review of provitamin A determination in plant foods (1989) J. Micronutr. Anal., 5, pp. 191-225Rodriguez-Amaya, Kimura, Godoy, Arima, Assessment of provitamin A determination by open column chromatography/visible absorption spectrophotometry (1988) J. Chromatogr. Sci., 26, pp. 624-629Sweeney, Marsh, Separation of carotene stereo-isomers in vegetables (1970) J. Assoc. Off. Anal. Chem., 53, pp. 937-940Sweeney, Marsh, Liver storage of vitamin A in rats fed carotene stereoisomers (1973) J. Nutr., 103, pp. 20-25Tsukida, Saiki, Sigiura, Structural elucidation of the main cis-β-carotenes (1981) J. Nutr. Sci. Vitaminol., 27, pp. 551-561Tsukida, Saiki, Takii, Koyama, Separation and determination of cis/trans-β-carotenes by high performance liquid chromatography (1982) J. Chromatogr., 245, pp. 359-364Vetter, Englert, Rigassi, Schwieter, Spectroscopic methods (1971) Carotenoids, pp. 189-266. , O. Isler, Birkhauser Verlag, BaselZechmeister, Stereoisomeric provitamin A (1949) Vitam. Horm., 7, pp. 57-81