SummaryWe have established the screening methods for PNP and ADA deficiencies and screened 41 immunodeficient patients since 1976. Two of 8 patients with combined immunodeficiency have been found to have ADA deficiency. They were the first and second cases in Japan.The second case with ADA deficiency, born of a second‐cousin marriage, was a 14‐month‐old boy when he was diagnosed as having variable type of combined immunodeficiency associated with ADA deficiency. His T‐cell function was impaired but immuno‐globulin titers were within normal range. His parents and five relatives including paternal grandfather, maternal grandmother, two maternal uncles and paternal aunt were shown to have reduced erythrocyte‐ADA activity. Their isozyme patterns were ADA 1–0.To characterize the heterozygous carrier, we used a new approach to analyze the ADA‐gene transmission. The theoretical values of the erythrocyte‐ADA activity obtained from the analysis were consistent with the measured ones.We propose that the heterozygous carriers of ADA deficiency must be determined not only by the assay of ADA activity but also by the analysis of the ADA