Twenty-five patients were studied to determine the effects of thermal injury on neutrophil bactericidal function and superoxide release and on lymphocyte proliferation. Neutrophils in patients with burns had depressed killing of Staphylococcus aureus for more than 150 days after burn injury, but killing of Escherichia coli returned to normal. FMLP-stimulated superoxide release by neutrophils in patients with burns was depressed for over 100 days after burn injury, whereas superoxide release by neutrophils in patients with burns stimulated with serum-opsonized zymosan was depressed for 42 days after burn injury. In patients with burns lymphocyte proliferation, with phytohemagglutinin as a mitogen, was suppressed for up to 85 days after injury, then returned to normal. The mixed lymphocyte response was suppressed up to 170 days after injury.