Recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) seeded in the lower compartment of chemotaxis chambers induced migration of human peripheral blood monocytes and polymorphonuclear phagocytes. rhGM-CSF was active in inducing phagocyte migration at concentrations as low as 10 CML units/ml (corresponding to 0.002 microgram), and maximal activity was observed at 10(2)-10(4)/ml. Checkerboard analysis performed by seeding different concentrations of rhGM-CSF above and below the filter revealed that maximal induction of migration required a positive concentration gradient between the lower and upper compartment, and that rhGM-CSF elicited an actual chemotactic response in phagocytes. Human umbilical vein endothelial cells and blood large granular lymphocytes (LGL) responded poorly or not at all (endothelial cells) to rhGM-CSF under conditions in which appropriate reference chemoattractants were active. The chemotactic activity of rhGM-CSF may serve to recruit phagocytic cells from the blood compartment to amplify resistance against noxious agents.