It is shown here that the C4-77 and C4-86 bone marrow clones with properties of pro-T lymphocytes and the Bc/Bm11 and CB/Bm7 clones with characteristics of pro-B lymphocytes grow in recombinant interleukin 4 (rIL4)/BSF-1 and IL3, but not in rIL2. The proliferative cell responses to rIL4/BSF-1 were always less that approximately 50% of those achieved by the clones in response to IL3. The CC11 monoclonal antibody (mAb) specific for IL3-sensitive mouse cells did not affect the action of rIL4/BSF-1, but it did inhibit the action of IL3 on the clones. The PC61 mAb against IL2 receptors had no effect on either rIL4/BSF-1- or IL3-driven responses. All four clones carry on the cell membrane the glycoproteins recognized by the CC11 mAb and by the PC61 mAb as assessed by immunofluorescence staining and flow cytometry. We conclude that the pro-T and the pro-B clones express functional receptors for IL3 and rIL4/BSF-1 and nonfunctional receptors for IL2, that rIL4/BSF-1 promotes growth of these clones via an IL3- and IL2-independent pathway and discuss the possible biological significance of these findings.