Following the observation that, besides acute myeloid leukemia cells, acute lymphoid leukemia cells of either B or T phenotype could express the transcript for the colony-stimulating factor 1 (CSF-1), a growth factor known to be restricted to the monocytic-macrophage lineage, various sources of resting and/or activated T cells and thymocytes were screened for expression of this hemopoietic growth factor. We report here that the CSF-1 transcript was rapidly (7 h) induced in T cells by a variety of stimuli, but was not detectable in either resting T cells or thymocytes. In addition, secretion of CSF-1 was detectable in the supernatants of activated T cells by 72 h, with a peak around 92-120 h. In contrast to activated monocytes, the transcript of the c-fms proto-oncogene, the product of which is the receptor for CSF-1, was not detectable in either resting or activated T cells. This observation could be relevant to the intimate relationships between T cells and antigen-presenting cells during immune responses.