CD38 is a 42 kDa membrane-associated ectoenzyme expressed by a large proportion of human and mouse lymphocytes. Agonistic antibodies to CD38 induce a strong proliferative response in lymphocytes additionally co-stimulated with other growth co-factors such as IL-4, IL-2 plus accessory cells or sub-mitogenic doses of endotoxin. We show here that B lymphocytes from unstimulated X-linked immunodeficient (xid) mice are unresponsive to CD38 stimulation, both in terms of proliferative response and surface antigen modulation. This CD38 unresponsiveness is evident in the presence of excess quantities of, and normal responses to, the accessory growth co-stimulants required for this response. CD38 molecules expressed on xid B cells are normal in terms of expression levels, size and enzymatic activity, suggesting that CD38 unresponsiveness reflects a down-stream signaling defect. In light of the recent proposal that the xid gene encodes a tyrosine kinase called Bruton's tyrosine kinase (btk), these data suggest that btk is either an integral component or an indirect regulator of the CD38-induced signal transduction pathway.