Enzymatic parameters of aminopeptidase-I that may be sensitive to temperature an solute variations were investigated to provide a functional explanation for specific activity differences among genotypes in natural populations. The effect of temperature on the apparent Km of L-leucyl-4-methoxy 2-naphthylamide and th dipeptide phenylalanyl-glycine was small, especially between 10 and 25 C. The apparent Km varied only between 36.7 and 49.8 microM at these temperatures and the six common genotypes did not differ in temperature-dependent substrate affinities. While pH had a significant effect on Km, no differences among genotypes were observed. Activation enthalpies were also identical among genotypes. Thermal inactivation was slowest at 15 C and the same for all genotypes. Of 18 tested amino acids, only phenylalanine inhibited aminopeptidase-I; K1 values ranged from 1.2 to 0.8 mM and were the same for all genotypes. Small differences among genotypes were detected in the inhibitory effect of zinc. The concentration of aminopeptidase-I enzyme was the same for all genotypes in a population exposed to oceanic salinity, but the concentration of Lap 94/94 was 15% lower than that of other genotypes in a population experiencing estuarine salinity. Genotypes with the Lap 94 allele exhibited higher apparent Kcat values in all population samples.The probable genotype-dependent effects of enzyme concentration and Kcat differences are discussed with regard to maintenance of the polymorphism and genetic differences among populations.