FDC-P1 is a murine myeloid cell line that requires interleukin 3 (IL3) for survival and proliferation. While the biological effects of IL3 have been well described, the biochemical mechanisms of IL3 actions have only recently been examined. We have investigated whether IL3 or PMA stimulates phosphorylation of proteins on tyrosine as well as on serine/threonine residues as previously described [(1986) Blood 68, 906-913; (1987) Biochem. J. 244, 683-691]. Here we report that both IL3 and PMA stimulate the tyrosine phosphorylation of at least two proteins: pp70 and pp50 in FDC-P1 cells.