The activity of total aldolase and of the isoenzymes aldolase A and aldolase B was determined in a sample of more than 100 healthy unrelated persons as well as in pairs of 20 monozygotic and 20 dizygotic twins. In monozygotic twin pairs the specific activity is almost identical, whereas significant differences were observed in dizygotic twins. Analogous results with respect to these twin studies were obtained by estimating the phenotypes of the phosphoglucomutase locus 3, arantitrypsin, alanine transaminase, the 3rd component of complement and transferrin.