Levamisole at concentrations of 10(-3) M or 10(-4) M consistently increased neutrophil random motility and chemokinesis (stimulated random migration). Similar concentrations also increased directional movement of polymorphonuclear leukocytes to both endotoxin-activated serum and hydrolyzed casein. This effect on chemotaxis was due to a true stimulation and was not due solely to increased random movement. The effect of levamisole on the neutrophils could be removed by washing, but persisted if the cells were initially treated with levamisole and serum or endotoxin-activated serum. After neutrophil stimulation with chemotactic factor an initial rise in intracellular cyclic AMP levels was detected which was not influenced by prior levamisole treatment. Intracellular cyclic GMP levels after an initial slight depression, returned to resting levels and gradually diminished over a 60-minute period. Levamisole-treated cells consistently showed higher cyclic GMP levels and it is postulated that by maintaining intracellular cyclic GMP levels, microtubular assembly and cell motility might be enhanced.