Recombinant murine GM‐CSF produced in Escherichia coli was purified to homogeneity and tested in parallel with purified native GM‐CSF. Both recombinant and native GM‐CSF stimulated granulocyte and/or macrophage colony formation by adult and fetalmouse progenitor cells, and with adult marrow cells the specific activity of the recombinant GM‐CSF (25 × 108 U/mg) was similar to that of the native form (15 × 108 U/mg). At high concentrations (> 200 U/ml), both forms of GM‐CSF also stimulated eosinophil colony formation by adult marrow cells and, at very high concentrations (> 800 U/ml), megakaryocyte and some erythroid and mixed‐erythroid colony formation. Recombinant GM‐CSF was as effective in stimulating the proliferation of the GM‐CSF‐dependent cell line FD as the native molecule. Both recombinant and native GM‐CSF were able to induce partial differentiation in colonies of WEHI‐3B myeloid leukemic cells. Recombinant GM‐CSF competed effectively for the binding of 125l‐labeled native GM‐CSF to hemopoietic cells, and anti‐serum to recombinant GM‐CSF also neutralized the biological activity of native GM‐CSF. The bacterially synthesized GM‐CSF was a slightly more effective stimulus for megakayocyte colony formation than then native molecule. The demonstration that purified bacterially synthesized GM‐CSF is biologically active in vitro now permits studies to be undertaken on the in vivo effects of this material.