Ribavirin, a broad spectrum antiviral agent, has been shown to exhibit immunosuppressive activity. This property has raised concerns during clinical trials because candidates for antiviral therapy are those who may develop secondary bacterial infection. We therefore investigated a number of parameters of neutrophil function after the in vitro addition of various concentrations of ribavirin. At pharmacologic concentrations (1.25-7.5 micrograms/mL), percent killing of phagocytized bacteria was increased as measured by an acridine orange fluorochrome microassay, significantly higher at a concentration of 5 micrograms/mL (p less than .02). There was no concomitant adverse effect on adherence, random migration, chemotaxis, opsonization or phagocytosis. The present data indicate that ribavirin has no acute suppressive effect on granulocyte function and may even enhance bacterial killing capabilities for treated patients.