We analyzed the proliferative response of the growth factor-dependent murine cell lines FDCp1, DA1-a, 32DC1, Ea3.15, 7TD1, BCL1 and of femural bone marrow cells for their sensitivity to various cytokines, viz. rhIL-1 beta, rhTNF, rhIL-2, mIL-3, rmIL-4, rmIL-5, rhIL-6, rhG-CSF and rmGM-CSF. We also tested for IL-1 and TNF-mediated cytokine secretion by several T cell lines and thymocytes. In all T cell systems, IL-1 alpha and IL-1 beta were equally active in the induction of cytokine production, except for the rat/mouse T cell hybridoma PC60. This cell line exhibited a 10-fold difference in specific activity for the induction of cytokine secretion between rhIL-1 alpha and the other human or murine IL-1 species. Furthermore, IL-1 and IL-2 synergistically induced PC60 cells to produce a factor, which was preferentially active on FDCp1-cells, provisionally called FDCp1-growth factor. SDS-PAGE analysis of partially purified FDCp1-GF showed 19 kDa and 24 kDa-associated biological activities. Amino-terminal and internal amino acid sequences of both bands were determined and on this basis, we identified FDCp1-GF as rat GM-CSF.