The effects of recombinant interleukin -3 (IL-3) on the myelosuppression induced by irradiation and cyclophosphamide (CY) were observed in mice. The experimental results were as follows: (1) Intraperitoneal (i.p.) or subcutaneous (s.c.) injections of rh IL-3 daily for 5 days immediately after irradiation could alleviate the radiation-induced hematopoietic injuries. The yields and the numbers of CFU-E, BFU-E, CFU-Mix and CFU-GM in femural marrow in mice on the 9th day post exposure to 7 Gy gamma-rays were much higher than those in control animals. Meanwhile, rh IL-3 showed a weak influence on the numbers of bone marrow nucleated cells (BMC) and endogeneous CFU-S. (2) Subcutaneous administration of rh IL-3 for 5 consecutive days brought a favour of CY treated mice to elevate the yields of hematopoietic progenitor cells. (3) The effects of rh IL-3 on myelosuppression induced by radiation or CY were closely related to the route of administration, administration schedules, dosage of this cytokine and the disease state as well. So it seemed important to research still further an appropriate, optimal and flexible guide for clinical use. (4) In vitro rh IL-3 had no effect on the growth of murine BMC and CFU-GM. In comparison with control, after coculture with rmIL-3 (recombinant mouse interleukin (3)) BMCs of normal mice and mice irradiated with 2 Gy gamma-rays proliferated more rapidly and the yield of CFU-GM in them were higher. (5) The mechanism of the effects of rh IL-3 in marrow hypoplastic mice might be related to the indirect promoting influences on the proliferation and/or differentiation of the radiation damaged hematopoietic progenitor cells and/or hematopoietic stem cells.