The effect of Cordyceps sinensis crystal (CS-Cr) on stimulating proliferation of erythroid progenitor cells (CFU-E and BFU-E) in LACA mouse marrow in vivo and in vitro by methyl cellulose gel culture system is reported. The numbers of CFU-E and BFU-E were increased after 5 consecutive daily treatment with 100, 150 and 200 mg/kg of CS-Cr with a peak at 150 mg/kg. Higher doses (> 150 mg/kg) of CS-Cr resulted in a reduction of the peak of CFU-E and BFU-E and then, the numbers returned to the control level with increased doses. The cytosine arabinoside (Ara-C) suicide test showed significant increases in the percentage of CFU-E and BFU-E in S-phase after CS-Cr treatment. Pretreatment of mice with CS-Cr could protect CFU-E and BFU-E against the cytotoxic agent--harringtonine. Addition of CS-Cr to culture system also promoted the generation of CFU-E and BFU-E at concentrations of 150-200 micrograms/ml in vitro. With a liquid culture technique, a stimulatory action of CS-Cr on fibroblast colony-forming units (CFU-F) proliferation was seen in vivo and in vitro.