Immature Glycine max (L.) Merrill cotyledons were cultured in a defined medium containing different nitrogen sources. Glutamine was the most efficient source in terms of protein accumulation in the cotyledons. Asparagine was less efficient (about 70 per cent that of glutamine) while allantoin was a poor source of nitrogen. This was also true for older cotyledons where asparaginase and allantoinase activities were maximal. The utilization of both asparagine and allantoin (but not glutamine) was totally inhibited by methionine sulfoximine suggesting that their metabolism involves ammonia assimilation via glutamine synthetase. Apparently, neither exogenous or endogenously-generated ammonia had much effect on glutamine utilization, but ammonia did have a small inhibitory effect on asparagine, which may in part account for the lower efficiency observed with this amide. © 1987 Annals of Botany Company.596597601