Evidence is presented which shows that alpha-toxin elaborated by Staphylococcus aureus can affect certain of the biological functions of human polymorphonuclear leukocytes (PMN). The pretreatment of PMNs with low doses (less than 10 hemolytic units) of purified toxin enhanced their ability to phagocytose and to kill serum-opsonized staphylococci. At higher doses (greater than or equal to 10 hemolytic units), a certain amount of cell damage was caused (detectable by trypan blue uptake), which resulted in a reduction of their phagocytic capacity. These effects were not seen either with heat-inactivated (60 degrees C, 30 min) or with antibody-neutralized toxin. It is thought that the toxin can bind onto the membrane of the PMN, either exposing additional receptor sites for opsonized bacteria or speeding up the transport of staphylococci across the membrane. Once ingested, the bacteria were more readily killed by the PMNs.