The cause of myelosuppression in hairy cell leukemia (HCL) has been ascribed to a reduction of the circulating progenitor cell (CPC) compartment and to suppression of hematopoiesis by TNF-a. The present study was performed to evaluate the inhibitory effect of hairy cells (HCs) and a possible lack of hematopoietic growth factors on the number of autologous CPCs in vitro. In initial experiments the number of circulating BFU-E, CFU-GM and CFU-mix in HCL patients was found decreased. Monocytopenia but not the number of circulating HCs correlated to the degree of colony reduction in our patients. This pointed to a lack of colony stimulating factors (CSFs) in HCL. Actually, the growth of BFU-E, CFU-GM, and CFU-mix improved upon the addition of IL-3 and GM-CSF in HCL patients but not in healthy donors. To test the suppressive role of HCs in our assay system, cultures were performed after removal of autologous HCs. The results showed that in HC-depleted cultures the numbers of BFU-E, CFU-GM, and CFU-mix were significantly higher. This inhibitory effect of HCs could partially be neutralized by the addition of monoclonal antibodies against TNF-a. When the assays were performed with the removal of HCs and the addition of CSFs normal progenitor cell counts were detected in most patients. We conclude that HCs mediate the inhibition of colony growth in part by TNF-a. Monocytopenia is related with a deficiency of CSFs in this disease. The reduced colony growth in HCL, therefore, is due to both the inhibitory effects of HCs and the deficiency of CSFs. We suppose that the CPC-compartment is actually preserved in this disease.