IL-3 and granulocyte-macrophage CSF are hemopoietic growth factors involved in monocytopoiesis and functional stimulation of circulating blood monocytes. We demonstrate that both cytokines enhance the adhesion of purified human monocytes to cultured human umbilical vein endothelial cells and to plastic surfaces. The stimulation seen was biphasic: an early phase detectable by 10 min, and a late phase seen after 9 h of in vitro culture. IL-3- and granulocyte-macrophage-CSF-stimulated adhesion was seen at concentrations as low as 6 pM, with maximal monocyte adhesion of up to 60% seen at concentrations of 60 pM and above. Both phases of stimulated adhesion were partially inhibited by a monoclonal antibody to CD18, the common beta-chain of the leukocyte functional Ag family of adhesion molecules, but not by an antibody to CD11b, the alpha-chain of MAC-1. However, a difference in the mechanism by which the early and late phases of stimulated adhesion arise could be shown by the use of cycloheximide as an inhibitor of protein synthesis. Although the late phase was totally dependent on de novo protein synthesis, early phase adhesion was not inhibited by cycloheximide, suggesting receptor redistribution or conformational change as the mechanism mediating enhanced adhesion at this time. These findings may be relevant to the pathogenesis of inflammatory disease and may have implications for the clinical use of these cytokines.