Adenosine deaminase (ADA) deficiency has been reported in association with severe combined immunodeficiency disease (SCID). The mechanism by which ADA deficiency causes immune dysfunction has been investigated in model systems to which the ADA inhibitor deoxycoformycin (dCf) had been added. Previously, we demonstrated that dCF did not prevent proliferation and differentiation of myeloid and lymphoid stem cells. We have now shown that addition of deoxyadenosine to dCf-containing cultures inhibited proliferation of hemopoietic stem cells. This inhibition was, however, equally effective for both normal myeloid and lymphoid stem cells. These findings suggest that other differences may exist between SCID myeloid and lymphoid stem cells to account for the relative sparing of myelopoiesis in SCID patients.