Abstract: Full length cDNAs, L2G25B and 4-1BB, were isolated and sequenced. The cDNA L2G25B encodes for the lymphokine, macrophage inflammatory protein-1&agr; or MIP-1&agr;. The studies disclosed herein suggest that MIP-1&agr; and MIP-&bgr; can, through rapid action, modulate early myeloid progenitor cell proliferation. Recombinant proteins have been produced for the cytokine, L2G25BP (Macrophage Inflammatory Protein-1&agr;, MIP-1&agr;). By employing the recombinant protein (rMIP-1&agr;), receptors for MIP-1&agr; were identified on Con A-stimulated and unstimulated CTLL-R8, a T-cell line, and LPS-stimulated RAW 264.7, a macrophage-cell line. Purified recombinant murine macrophage inflammatory protein-1 alpha (rmuMIP-&agr;), was assessed for effects on proliferation of granulocyte-macrophage (CFU-GM), erythroid (BFU-E), and multipotential (CFU-GEMM) progenitor cells. The results suggest that rmuMIP-1&agr; has myelosuppressive activity in vivo.