Mouse IL-5 (mIL-5) acts on B cells and eosinophils to induce growth and differentiation through the mIL-5 specific receptor (mIL-5R). The functional high-affinity mIL-5R is a heterodimer composed of alpha and beta chains. We investigated the expression of mIL-5R and the responsiveness of B cells and eosinophils to mIL-5 in X-linked immunodeficient (xid) mice. mIL-5R expression analyzed by using mAbs specific for alpha and beta chains revealed that xid B cells had fewer mIL-5R alpha +mIL-5R beta + than BALB/c B cells. In particular, a decrease in the number of peritoneal mIL-5R+ B cells among Ly-1 B cells (known as B-1 cells) was remarkable. Furthermore, the frequency of precursors of mIL-5 responsive B cells in xid mice was approximately 100-fold lower than that of BALB/c mice. Interestingly, sorted mIL-5R+ peritoneal B cells from xid mice displayed a low response to mIL-5. Intraperitoneal injection of mIL-5 into BALB/c mice induced polyclonal IgM production and an increase in the number of eosinophils. The same regimen failed to induce an increase in the same parameters in xid mice. However, xid mice showed mIL-5-induced eosinophilia in peripheral blood to a similar extent as BALB/c mice. Eosinophils from mIL-5-injected xid mice expressed both alpha and beta chains of mIL-5, and responded to mIL-5 with prolonged in vitro survival.