Possible selective interaction between genetic polymorphisms of acid phosphatase locus 1 (ACP1) and adenosine deaminase (ADA) has been investigated in a sample of 211 infants from diabetic women, and in 350 consecutive infants from normal women. Newborns from diabetic pregnancies carrying the ADA2 allele show a lower proportion of BA and CB phenotypes (heterozygotes for the main allele of ACP1 system), compared with both their mothers and normal infants. The observation suggests that, in a diabetic environment, intrauterine selection may act against double heterozygotes for the ACP1 and ADA systems.