We have used the ability of opsonised bacteria to stimulate luminol enhanced chemiluminescence of human neutrophils to examine the opsonic capabilities of normal and hypogammaglobulinaemic sera for four common bacterial pathogens. Preparations of human immunoglobulin modified for i.v. use have then been compared with unmodified Cohn Fraction II for their effectiveness in improving opsonization when added to antibody deficient sera in vitro. Hypogammaglobulinaemic sera exhibited impaired opsonisation of Haemophilus influenzae, and severely antibody deficient sera also opsonized Streptococcus pneumoniae and Pseudomonas aeruginosa poorly. The opsonization of these organisms was improved by Cohn Fraction II, and by pH 4 and beta-propionolactone treated immunoglobulins, in descending order of effectiveness. Pepsin digested immunoglobulin was inactive, and in some cases impaired opsonic capacity. The opsonisation of Staphylococcus aureus by hypogammaglobulinaemic sera was near normal, and was not improved by any immunoglobulin. This technique, which assesses biological activity of immunoglobulin, is useful in comparing preparations, and may help to establish appropriate dosage and frequency for intravenous immunoglobulin replacement therapy.