Quantitative variations of blood cell progenitors were studied in thrombocytopenic post-chemotherapy patients and in healthy plateletpheresis donors with the aim of better understanding the reasons for their varying presence in peripheral blood. In 6 post-chemotherapy patients with severe thrombocytopenia on days 21-27 after initiation of chemotherapy when white blood cell counts were approximately 2.1-6.0 x 10(9)/l, a balance was observed between the most and least immature progenitors: levels of CFU-Meg were significantly lower than control values whereas levels of CFU-GEMM were 2-fold higher than in controls. Results suggest that an activator distinct from the known poietins may stimulate very immature progenitors. In 15 plateletpheresis donors, numbers of CFU-GEMM and CFU-Meg were greatly increased, respectively 5.5-fold and 10-fold, following plateletpheresis. Data once again indicate a major role of CFU-GEMM in the production of mature blood cells. As the most immature progenitors and thrombocytopoiesis stimulating factor(s) are both present in peripheral blood, such factors may be responsible for the initial engagement of these very early progenitors into a specific cell line.