The effect of haem arginate, a new stable haem compound, was studied on colony formation by erythroid (CFU-E, BFU-E) and granulocyte-macrophage (CFU-GM) progenitors from the bone marrow of 12 healthy persons. At the concentration of 10 mumol/l haem arginate significantly (p less than 0.05) increased and at the concentration of 200 mumol/l significantly decreased (p less than 0.005) the colony formation of CFU-E. A similar, though not significant, trend was seen in the BFU-E colony growth, while the CFU-GM colony formation was not influenced at any concentration between 1 and 200 mumol/l. We conclude that the stimulation of erythropoiesis in vitro is seen at lower concentrations with haem arginate than with former haemin preparations.