Endogenous histamine regulates the haematopoiesis. Histidine decarboxylase inhibitor decreases the histamine level, and its intracellular antagonist decreases the histamine effect. The effect of histidine decarboxylase inhibitor (α-fluoromethyl histidine) and the intracellular antagonist of histamine [NN-diethyl-2-4-(phenylmethyl) phenoxyethan-amine-HCl] was investigated on the colonyforming ability of human peripheral progenitor cells. Semi-solid culture medium was used both in the presence and in the absence of 3 U / ml erythropoietin. α-Fluoromethyl histidine was used in the range of 50 through 150 μ Mol / ml, the concentration of NN-diethyl- 2-4-(phenylmethyl) phenoxyethanamine-HCl was between 5 and 25 μ Mol / ml. The number of both the erythroide and the granulocyte macrophage colony was significantly decreased in a concentration dependent manner by the presence of both NN-diethyl-2-4-(phenylmethyl) phenoxyethanamine-HCl (in all concentrations used) and α-fluoromethyl histidine (at higher concentration). The inhibitory effect was decreased by erythropoietin.