Interleukin-3 (IL-3)-induced activation of endogenous Rac-1, Rac-2, and Cdc42. Rac-1 was also activated by colony-stimulating factor-1 (CSF-1), Steel locus factor (SLF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and IL-5 or by cross-linking the B-lymphocyte receptor for antigen (BCR). The activation of Rac-1 induced by cross-linking the BCR or by IL-3 stimulation was blocked only partially by Ly294002, with about 25% to 30% of Rac-1 activation still occurring in the absence of detectable increases in phosphatidyl-inositol-3 kinase (PI-3K) activity. Overexpression of constitutively active mutants of H-Ras, N-Ras, or M-Ras resulted in activation of coexpressed Rac-1 through an Ly29402-resistant, PI-3K-independent mechanism. Overexpression of constitutively active mutants of p21 Ras, or Rac-1, but not of PI-3K, was sufficient for activation of p38 mitogen-activated protein kinase (MAPK) in cells of hemopoietic origin. Inhibition of increases in PI-3K activity by Ly294002 had no effect on the IL-3-induced activation of p38 MAPK. In contrast, Ly294002 partially inhibited the activation of p38 MAPK induced by cross-linking of the BCR, although some p38 MAPK activation occurred in the absence of increases in the activity of Rac-1 or PI-3K. The activation of Rac-1, Rac-2, and Cdc42 by IL-3 and other hemopoietic growth factors is likely to be an important component of their actions in promoting growth, survival, and function.