Standard preparations of immunoglobulin for intravenous use consist predominantly of IgG (greater than 95%). We have compared the ability of a standard preparation (Sandoglobulin) with that of a new preparation (Pentaglobin, containing 12% IgM and 12% IgA) to improve the opsonic activity of antibody-deficient human sera in vitro. Panhypogammaglobulinaemic sera were poorly opsonic for five of six organisms tested, particularly Pseudomonas aeruginosa, Escherichia coli and Streptococcus pneumoniae, but opsonized Staphylococcus aureus almost normally. Both immunoglobulin preparations significantly improved the opsonic activity of the antibody-deficient sera for most organisms. The major difference between the two preparations was the ability of Pentaglobin to supply opsonins for P. aeruginosa, E. coli and Klebsiella pneumoniae, while Sandogloblin was significantly more potent in opsonins for Haemophilus influenzae. Pentaglobin demonstrates significant in vitro opsonic activity, particularly for enterobacteria (coliforms) and P. aeruginosa. Its content of IgM antibodies appears to confer special properties on Pentaglobin not seen with standard preparations of immunoglobulin for intravenous use. Its place in clinical practice remains to be determined but it may have a possible role in augmenting host defence mechanisms in Gram-negative septicaemia.