The mitogenic effect of corticotropin-releasing factor (CRF) on rat lymphocytes was investigated. When rat splenocytes were cultured for 48 hr with CFR, a dosedependent increase in incorporation of 3H-thymidine (3H-Tdr) was observed, with a maximal response at 10 nM CRF. Comparison of the proliferative effect of CRF on enriched populations of B lymphocytes, T lymphocytes, or macrophages revealed that only B lymphocytes responded following treatment with CRF. When lymphocytes derived from different lymphoid tissues were compared, CRF had a greater proliferative effect on lymphocytes derived from gut-associated lymphoid tissue (mesenteric lymph nodes and Peyer's patches) than on lymphocytes from spleen or inguinal lymph nodes; CRF had no effect on thymocytes. Synthetic fragments of CRF were used to determine which portions of the peptide are recognized by lymphocytes. The C-terminal fragments a- helical CRFM1 and CRFZla1 were as potent as native CRF in stimulating B-lymphocyte proliferation, whereas CRFI-** did not stimulate proliferation. The activity of these peptides suggests that CRF stimulates lymphocyte proliferation by cellular recognition of structural determinants in the C-terminal one-half of the peptide.