A retroviral vector called SAX, containing the cloned human cDNA for adenosine deaminase (ADA), has been constructed and used to introduce the ADA gene into cultured T- and B-lymphocyte lines derived from patients with ADA deficiency. DNA analysis showed that the SAX vector was inserted intact into the T and B cells at approximately one copy per cell. The treated cells produced the characteristic isozymes of human ADA at a level similar to normal T and B lymphocytes. It is known that ADA-deficient lymphocytes are unusually sensitive to high levels of 2'-deoxyadenosine, and this is the mechanism thought to underlie the selective lymphocytotoxicity associated with ADA deficiency in vivo. Expression of the introduced ADA gene was sufficient to reverse the hypersensitivity of these genetically deficient lymphocytes to 2'-deoxyadenosine toxicity. These results support the suggestion that retroviral vector gene-delivery systems show promise for application to human gene therapy.