Thymohexine treatment of intact CFU-S was shown not to change their capacity to form colonies both in vivo and in vitro. Thymohexine, similarly to thymus cells, increased colony yield from rabbit antimouse brain antiserum (RAMBS)-treated bone marrow. Combined treatment with thymohexine and thymocytes did not restore colony formation by RAMB serum-treated bone marrow. Thymohexine halved the harmful effect of low doses of ionizing radiation on CFU-S, and, moreover, treatment with the peptide after 4 Gy-irradiation led to more intensive restoration of CFU-S population as compared to control.