A sequential, prospective analysis of humoral and cellular immune function was performed on 20 burn patients with injuries involving ≤45% total body surface area. Infected patients had significantly worse neutrophil bactericidal activity against Staphylococcus aureus 502A than did noninfected patients. Chemotaxis of neutrophils correlated poorly with infection although chemotaxis was frequently abnormal. The opsonic index of serum was depressed early after the burn but returned to nearly normal values by the fourth to the fourteenth postburn day. There was no difference between infected and noninfected patients. Serum levels of IgG, properdin and C3, while initially low, remained within the normal range after the ninth postburn day in both groups. Factor B levels rose rapidly during the first three weeks after injury to more than double normal levels in many patients. Suggestive evidence for consumption of opsonic protein occurred with five of 19 episodes of bacteremia. The responsiveness of isolated lymphocytes to PHA was normal. However, patients' sera were shown to significantly inhibit the responsiveness of normal lymphocytes to PHA. Analysis of immunologic profiles for individual patients indicates that abnormalities of neutrophil function are the most important acquired defect predisposing patients to the development of bacteremia following major thermal injury; abnormalities of opsonic action play a secondary but important role.