Canine X-linked severe combined immunodeficiency disease (XSCID) is characterized by a failure to thrive, thymic dysplasia, and a lack of a T lymphocyte mitogenic response. As in human XSCID, affected dogs in our colony have a mutation in the IL-2R-gamma gene. This mutation dramatically altered T lymphocyte development, because XSCID thymi were severely reduced in size and cellularity, contained an increased proportion of immature CD4-CD8- thymocytes, a decreased proportion of intermediate CD4+CD8+ thymocytes, and a normal proportion of CD4+CD8- and CD4-CD8+ thymocytes. XSCID thymi were also deficient in the percentage of CD3-L+ thymocytes. Interestingly, several XSCID dogs had normal percentages of CD3-L+ PBL. Although the mutation did not interfere with IL-2 production, PHA-activated XSCID PBL demonstrated severely diminished IL-2 binding and were nonresponsive to IL-2. These results indicate that the lack of a functional IL-2R-gamma chain in dogs with XSCID primarily affects developing CD4-CD8- thymocytes as they acquire the cell surface Ag CD3-L and interferes with the ability of peripheral T lymphocytes to bind and respond to IL-2.