The cell cycle inhibition of pluripotent bone marrow stem cells (CFU-S) by a fetal calf bone marrow extract (BME) was studied. The normally quiescent cells were made to enter cell cycle by treatment with either irradiation or the phase specific drug cytosine arabinoside (Ara-C). Using the vitro-vivo system of Frindel et al., we have shown that when the BME was added to the incubation medium, no triggering of CFU-S into cycle could be observed. The inhibitory effect was also demonstrated when the BME was injected simultaneously with the drug. The possible relationships of BME with stimulating factors controlling CFU-S entry into cycle are discussed.