ALTHOUGH a strict correlation between prostaglandins and the cyclic AMP system has been suggested for several cell types, and the mitogenic effect of cyclic AMP has also been proved1,2, the mitotic effect of prostaglandins has been demonstrated only for some cell systems3. Also, little is known about the haemopoietic effect of prostaglandins. As proved by Dukes4, several prostaglandins, especially PGE2, enhance the 59Fe and 14C-glucosamine incorporation into bone marrow cells and potentiate the effect of erythropoietin, in vitro. We have studied the effect of PGE2 on the proliferative state of haemopoietic stem cells (spleen colony forming units:CFU).