Gene frequencies of the red cell acid phosphatase enzyme system were estimated in a sample of 852 unrelated adult blood donors living in Copenhagen and were found to be PA = 0.372, PB = 0.551, PC = 0.077. Phenotypes were deter mined in 447 families with a total of 1,671 children. Deviation from Mendelian segregation was observed in A × CA matings when the sexes of the offspring were considered separately.