The ability of blood neutrophils of 18 serologically confirmed influenza patients to ingest killedStaphylococcus epidermidiswas testedin vitro, and compared with 15 controls, to determine whether defective phagocytosis might contribute to superimposed bacterial infections in influenza. Phagocytosis was normal at the acute phase of disease, but two weeks later the activity was significantly increased; this was not due to serum factors, and probably the cells themselves were unusually active. Normal ingestion of bacteria by blood neutrophils at the acute phase does not suggest defective phagocytosis as a cause of bacterial complications in influenza, but a local impairment in respiratory organs cannot be excluded.