The interaction of pulmonary surfactant protein A (SP-A) with influenza A HINI and H3N2 viruses was investigated. SP-A is a sialated C type lectin with affinity for mannose residues. Flow cytometry showed that binding of fluorescein isothiocyanate (FITC)-labeled SP-A to H3N2 virus-infected cells was specific and time- and concentration-dependent. Oligosaccharides did not affect the binding of FITC-SP-A to the infected cells. Preincubation of HINI and H3N2 with SP-A resulted in a dose-dependent reduction of the infectivity of the viruses to cells. Removal of the carbohydrate moiety of SP-A by N-glycosidase F or cleavage of its sialic acid residues by neuraminidase prevented the interactions of SP-A with the viruses. It is concluded that SP-A bind- to influenza A viruses via its sialic acid residues and, thereby, neutralizes the virus.