It is shown that neokyotorphin (the alpha-globin fragment 137-141) stimulates proliferation of normal cells (murine embryonic fibroblasts, red bone marrow and spleen cells) and tumor cells (murine melanoma and transformed fibroblasts L929) in the absence or in the presence of fetal bovine serum. In contrast to serum deprivation conditions, the ability to potentiate L929 cell growth in the presence of fetal serum is strongly cell density dependent. The peptide also enhances the viability of L929 cells, murine embryonic fibroblasts and of the primary cultures of murine red bone marrow cells and splenocytes under serum-deprivation conditions for at least 72 h. The results of flow cytometry analysis suggest that the effect of neokyotorphin on survival of L929 cells in serum-free culture medium is due to maintenance of cell proliferation in the absence of growth factors. Along with cell cycle progression the peptide induces reversible reduction of L929 cell size.