The purified amphipathic proteins, alpha s 1-casein, beta-casein, and alkali-denatured serum albumin were studied for chemotactic and enzyme-releasing effects on human neutrophil leucocytes. Evidence for chemotaxis both in fluid-phase gradients and on solid-phase gradients was obtained using visual assays. In fluid-phase gradients, neutrophils showed good orientation to gradient sources of these proteins at concentrations of 10(-4) to 10(-5) M. Solid-phase gradients of casein and of denatured albumin were prepared on glass coverslips, and the locomotion of neutrophils attached to these coverslips was filmed by time-lapse cinematography. Displacement of neutrophils towards the highest concentration of substratum-bound protein was observed, suggesting that neutrophils can show true chemotaxis on a solid-phase gradient. All three proteins induced enzyme release from neutrophils in the absence of cytochalasin B. Lysozyme release was equivalent to that released by stimulation with formyl methionyl peptide in the presence of cytochalasin B, but the proteins stimulated a smaller release of beta-glucuronidase than the peptide. The proteins stimulated release of neutrophil proteases which were able to digest both casein and denatured albumin extracellularly. It is suggested that this proteolytic activity may assist locomotion of neutrophils, especially on solid-phase protein gradients, by cleaving membrane-attached protein, thus both freeing cell-surface receptors and allowing the cell to detach itself from the substratum and continue movement.