Prospective sequential studies of the anti-bacterial function of neutrophils, lymphocyte responsiveness, opsonic capacity of serum and serum levels of C3b [b fragment of complement component 3], properdin, factor B, Ig[immunoglobulin]G and albumin were made in 32 patients with severe burn injury (.gtoreq. 45%), 21 patients with severe multisystem traumatic injury, 20 high-risk, infected patients and 22 renal transplant patients. Episodes (55) of bacteremia occurred in 37 of the 95 patients. Abnormal neutrophil function was clearly associated as a predisposing factor to these episodes whereas there was no association between bacteremia and low serum levels of C3, IgG, factor B or properdin. C3, factor B and IgG usually rose following bacteremia as acute phase proteins but there was evidence of a consumptive opsoniopathy in 11% of episodes. Defective opsonization was associated with a high risk of bacteremia only when there was a coexisting abnormality of neutrophil function (88% of such patients became bacteremic). None of 27 non-burned patients tested with delayed hypersensitivity antigens responded normally and there was regular depression of lymphocyte responsiveness to phytohemagglutinin-A and concanavalin A in a whole blood assay related to serum immunosuppressive factors, but poor responsiveness was not associated with bacteremia.