Two human choriocarcinoma cell lines were shown to be deficient in adenosine deaminase (ADA; adenosine aminohydrolase, EC 126.96.36.199) such that they did not produce bands on starch gels after electrophoresis and histochemical staining. Radiometric assay indicated that their ADA specific activity was approximately 2% that of HeLa (human) cell controls. Subclone analysis of one of the lines indicated that this deficiency was representative of individual cells of the line. After fusion of these cells with mouse fibroblasts having high ADA activity, most independently isolated hybrid clones expressed one of two, or both, additional (to the mouse) bands of ADA activity after electrophoresis. The expression of these extra bands in hybrids was dependent upon actual fusion. The phenomenon was observed in 30 of 45 independently derived hybrid clones from four different fusion experiments involving two different parental lines from each species. The pattern of appearance of the extra bands in independent hybrid clones and the tendency of a hybrid clone to lose one of the extra bands through subsequent passages suggests that the bands were the products of human genetic material. The extra bands electrophoretically comigrated with human ADA 1 and 2 from human ADA-1-2 heterozygotes and the faster-migrating of the two extra bands comigrated with human ADA 1 from HeLa cells. Therefore, we suggest that the bands appearing in hybrids are the products of the 1 and 2 alleles of the human ADA locus. The human cells used for fusion were deficient in ADA activity but contained the genetic information for ADA 1 and 2. Fusion with mouse cells having ADA activity resulted in the activation of both human gene products coded for on separate homologous chromosomes. We conclude that the human ADA locus is under manipulatable genetic regulation.