We administered recombinant human IL-1 beta (400 ng/d, s.c.) for 10 d to normal C57B1 mice and determined daily granuloid and erythroid parameters in marrow, spleen and blood. In the marrow CFU-GM numbers were not affected but later granuloid cell stages were moderately enhanced (170%). In the spleen, however, CFU-GM numbers were sharply increased (1600%), whereas the granuloid precursors only doubled. Blood granulocytes increased transiently to 275% on day 5. In the marrow all erythroid parameters were severely reduced. This reduction was partially compensated by the spleen where initially only BFU-E and with some delay also more mature erythroid cells accumulated. At the end of the treatment mice were slightly anaemic. When mice were treated with IL-1 and erythropoietin (10 U/d) simultaneously, the inhibitory effects on erythropoiesis were less severe. In agreement with in vivo results, IL-1 inhibited in vitro colony growth of CFU-E from normal bone marrow and spleen but spleen CFU-E from 5 d IL-1 treated mice were insensitive. We conclude that IL-1 can induce stimulation or inhibition of haemopoietic progenitor cells depending on their microenvironment.