During a paternity test ‘opposite’ types of red cell adenosine deaminase (ADA) were found in one family. The mother was of type ADA 2, three of her children were diagnosed ADA 1. Assays of enzyme activity revealed decreased ADA activity in these propositi. The mean activity was 60% of a control group. The existence of a silent allele segregating codominantly is assumed. The silent allele is designated ADA°.