GM-CSF is a potent stimulator of haematopoietic cells as well as some functions of granulocytes and macrophages. GM-CSF has many clinical uses; however, little is known about the effects of GM-CSF treatment in vivo on the responses of tissue lymphocytes in terms of secretion of Th-1 and Th-2 cytokines. We investigated this issue by measuring the responses of spleen cells from mice 24 h after treatment i.p. with saline or rmGM-CSF. GM-CSF at 16·7–50·0 µg/kg significantly increased (P < 0·01) spleen cellularity 2–2·5-fold and enhanced proliferative responses of non-stimulated (no mitogen) as well as concanavalin A (Con A)-stimulated spleen cells. Secretion of IFN-γ by Con A (2·5 µg/ml)-stimulated spleen cells was significantly (P < 0·01) increased from 1·8 µg/ml by control spleen cells to 5·2 μg/ml by GM-CSF spleen cells. IL-10 production was greater (0·25 μg/ml, P < 0·05) by Con A-stimulated spleen cells from GM-CSF-treated mice compared to control spleen cells (0·06 μg/ml). By contrast, there were no significant differences in IL-4 production by Con A-stimulated spleen cells from the different groups. These results show that GM-CSF treatment increases spleen cellularity and primes lymphocytes for enhanced responses. The enhanced production of Th-1 cytokines by primed lymphocytes may partially explain the beneficial role of in vivo administration of GM-CSF in several clinical situations.