The IgG immunoglobulin preparation Polyglobin® failed to enhance the combined phagocytic and serum bactericidal activity of 65 vol% of fresh defibrinated human blood from healthy adults against representative test strains of Serratia marcescens, despite documented O-agglutinin activity of Polyglobin against every test strain employed. It was concluded that pooled human IgG immunoglobulin, i.e. ‘natural’ IgG antibodies, would not be expected to exert a beneficial effect against S. marcescens in patients with sufficient residual, functional ‘natural’ antibodies and co