To all the volunteers of the study, to Tatiana Cristina Vieira de Carvalho for helping with data analysis, to the military personnel that helped in the field work and to the entire staff of Reference Center for Leishmaniasis – IPEC/FIOCRUZ-RJ. This work received support from FIOCRUZ, the National Council of Research- Brazil (CNPq), The National Foundation of Health in Rio Grande do Sul State and the General coordination of Laboratories, SVS–Ministry of Health, Brazil.A randomized double-blind trial was performed to determine the frequency of positive reactions to the Montenegro antigen (leishmanin) preserved in thimerosal (Merthiolate™) 1:10,000 or phenol 0.4%. The respective products were tested separately in 400 young healthy individuals from a non-endemic area for Leishmaniases. Each volunteer received one of the following reagents: merthiolated antigen, phenolated antigen, merthiolated saline, or phenolated saline. The frequency of positive responses to each reagent after the first application was as follows: 0% (phenolated saline), 9.2% (merthiolated saline), 34.6% (antigen in phenolated saline), and 41.1% (antigen in merthiolated saline). After 1 week, volunteers who had tested positive for merthiolated or phenolated antigen were retested with the respective preservative, while negatives were retested with the preservative they had not received during the first test. In all, 331 volunteers who received merthiolated saline during the study, of whom 41 (12.4%) tested positive. Meanwhile, 326 volunteers who received phenolated saline, 4 (1.2%) tested positive. Positive reactions in each group were similar in relation to gross appearance skin reactions. Considering the high frequency of hypersensitivity to thimerosal in the study population, it is recommended that this compound should be replaced as a preservative of the leishmanin antigen. Almost 30% of positive reactions to Montenegro antigen in what is considered a non-endemic region was surprising and will be the object of future studies.2026