In a forensic case of disputed paternity an apparent mother/child incompatibility with respect to red cell acid phosphatase was found, the mother appearing as type A and the child as type B. Determination of electrophoretic type and of acid phosphatase activity in 8 of the family members strongly suggested the presence of a silent gene P° in 4 of the individuals. The phosphatase levels in the four heterozygotes were about half the values expected from normal values determined in 100 healthy adults representing the different phenotypes. The distribution of red cell acid phosphatase types in 3,735 unrelated Danish adults and in 1,109 mother/child pairs is reported; gene frequencies Pa = 0.369, Pb = 0.566 and Pc = 0.065. The P° gene frequency was roughly estimated as 0.001. Results are reported on the application of the red cell acid phosphatase system to 300 2-men cases of disputed paternit