Summary The effect of C. parvum on in vitro colony-forming cell was evaluated. A single ip injection of the vaccine had no effect during the first 4 hr either on colony stimulating factor or on colony-forming cells. The enhancement of the colony-forming capacity of the bone marrow by C. parvum occurred after two injections of the vaccine within 48 hr. The stimulation of colony formation by C. parvum was time related function and occurred 5 days after the initiation of the cultures. The results of this study indicate that C. parvum appears to be a stimulant of bone marrow cells capable of proliferation and may be used as a valuable adjuvant against myelosuppressive agents.