We have studied effects of two partially purified human leukocyte (alpha) interferon (IFN) preparations (PIF-A and PIF-B) and a highly purified fibroblast (beta) IFN on the functional activity of normal human neutrophils (PMNs). In vitro, PIF-B conferred a significant and dose-dependent enhancement of chemiluminescence (CL) induced both by phagocytosis and a soluble stimulus, f-Met-Leu-Phe, and decreased killing of Staph. aureus. In contrast, PIF-A caused only a slight inhibition of bactericidal activity and had no effects on CL. beta-IFN had no effects on either bactericidal activity or CL. Migration under agarose was decreased with all of the IFN but phagocytosis and release of enzymes was not affected. PMNs from seven patients treated with PIF-A for multiple myeloma exhibited increased CL responses but no other PMN functions were affected. The findings that human IFN preparations affect PMN functions indicate that high-dose IFN therapy of immunocompromised patients should be carefully evaluated for the possibility of increased infectious complications.