BDF1 mice were rendered severely thrombocytopenic by antiplatelet serum and the effect of serum from these thrombocytopenic mice on early precursors of syngeneic bone marrow was studied in Dexter's long-term cultures. The CFUMega content had increased on the first day, reaching its maximum on the second and third days, and returned to its initial value by the seventh day. Repeated treatment prolonged the effect, and larger doses produced larger increases in CFUMega. Besides the increase in CFUMega content, the number of CFUS and of total cells also increased. The CFUC number did not change.