In this paper we demonstrate that IL-3 can act as a cofactor for the growth of some CD4+ T cells. This lymphokine synergized with IL-4 to induce both a unique set of protein tyrosine phosphorylations and the vigorous proliferation of the keyhole limpet hemocyanin-specific and I-Ab-restricted CD4+ Th0 cell clone, E6. In addition, neutralizing anti-IL-3 Abs specifically inhibited the growth of E6 T cells to Ag or anti-CD3 mAb stimulation. Finally, this T cell clone was shown to express both the IL-3R alpha-chain and an IL-3R beta-chain (AlC2A). An examination of other CD4+ T cell clones determined that one Th1 clone (A.E7), two Th0 clones (16B.2 and L9A.1), and one Th2 clone (D10.G4.1) were not influenced by the addition of rIL-3. However, proliferation of the Th2 clones CDC25 and CDC35 to CD3-stimulation was significantly enhanced by IL-3. The sensitivity of these latter two clones to IL-3 was also found to be associated with expression of IL-3R alpha-chains. Because E6 T cells are highly dependent on IL-4 for autocrine growth similar to Th2 cells, these results suggest that IL-3 may synergize with IL-4 to enhance the proliferation of a subset of IL-4-dependent CD4+ T cells, and the study indicates that IL-3R alpha-chain expression may be a specific marker of this CD4+ T cell subset.