The Ask1-LT19 mutant of maize, which contains a lysine insensitive aspartate kinase was repeatedly backcrossed to the mutant opaque-2 and its wild type counterpart, the inbred line Cat100-1. Aspartate kinase was partially purified and characterized from seedlings, anthers and 20- to 25-day old endosperm tissue isolated from the wild type and Ask1-LT19 mutant. In addition, aspartate kinase was also characterized from 20- to 25-day old endosperms isolated from the opaque-2 mutant and the double mutant Ask1-LT19/opaque-2. No major variation was detected in the sensitivity of aspartate kinase to feedback inhibition by threonine in the three tissues of all the genotypes tested. On the other hand, there was clear evidence of a reduction in sensitivity of aspartate kinase to inhibition by lysine, when the wild type was compared with the Ask1-LT19 mutant. The reductions in lysine sensitivity in the Ask1-LT19 seedlings, anthers and endosperm were 37, 41 and 25%, respectively. When the opaque-2 mutant was compared with the double mutant Ask1-LT19/opaque-2, there was a 54% reduction in the sensitivity of the double mutant endosperm aspartate kinase to lysine. The data suggest that the gene encoding the lysine-sensitive aspartate kinase (Ask1) may be regulated by the opaque-2 mutation in maize.