Gibbon interleukin-3 (rlL-3) has recently been cloned and found to have a high degree of homology with the human IL-3 molecule. In this investigation, we evaluated the effects of gibbon rlL-3 on normal human peripheral blood megakaryocyte progenitor cell growth in vitro. Gibbon rlL-3 exhibited substantial megakaryocyte colony stimulatory activity (Meg-CSA), supporting peak colony numbers at a concentration of 1 Uiml. Megakaryocyte colony growth induced by rlL-3 reached 58% of the maximum achieved with the active, Mcg-CSAcontaining protein fraction of aplastic canine serum. Increasing gibbon rlL-3 concentrations also stimulated a 4-5-fold increase in megakaryocyte colony size and resulted in a decrease in geometric mean megakaryocyte ploidy. Ploidy values fell from 8.5N ? 1.4 (*SEM) at an rlL-3 concentration of 0.1 Uiml to a minimum of 2.9N * 0.3 at 10 Uiml. In the presence of rlL-3 at 1.0 Uiml, megakaryocyte colony growth was linear with cell plating density and the regression line passed approximately through the origin. The effects of rlL-3 on megakaryocyte colony growth were independent of the presence of T-lymphocytes in the cultures. Cross-species evaluation of murine and gibbon IL-3 indicated that its bioactivity is species restricted. Murine IL-3 did not support colony growth from human megakaryocyte progenitors and gibbon rlL-3 showed no activity in stimulating acetylcholinesterase production by murine bone marrow cells. Gibbon rfL-3 is a potent stimulator of the early events of human megakaryocyte progenitor cel I development promoting predominantly mitosis and early megakaryocytic differentiation.