In the present study we used flow cytometry to investigate the phagocytosis of fluorescein isothiocyanate-labeled herpes simplex virus type 1 (FITC-HSV-1) by rat alveolar macrophages and the effects of surfactant protein A (SP-A) on this process. The phagocytosis of FITC-HSV-1 by alveolar macrophages, which was studied as a model for virus phagocytosis in general, was strongly enhanced in the presence of SP-A. The SP-A-mediated phagocytosis was time and concentration dependent, reaching a maximal level after 15 min of incubation and at an SP-A concentration of 5 micrograms/ml. Using a fluorescence quenching technique, we could show that at least 65% of the viruses were indeed internalized by the macrophages. The addition of SP-A to the system was sufficient for the phagocytosis of FITC-HSV-1 by the alveolar macrophages, suggesting that SP-A acts as an opsonin. This hypothesis was further strengthened by the observation that F(ab')2 fragments of immunoglobulin G directed against SP-A could abolish FITC-HSV-1 phagocytosis by alveolar macrophages preincubated with SP-A. Comparing the opsonic capacity of serum and SP-A, SP-A proved to be twice as potent as serum in stimulating phagocytosis of FITC-HSV-1 by alveolar macrophages. Complement factor C1q, which is known to possess a similar collagen-like domain as SP-A, did not stimulate phagocytosis of FITC-HSV-1 by alveolar macrophages nor did it inhibit SP-A-mediated HSV-1 phagocytosis. This study demonstrates that SP-A may play an important role in the antiviral defenses of the lung.