Since the most common site of infection in burned patients is the burn wound, we have previously studied the biologic effect of burn wound blister fluid (BF) on control lymphocyte and neutrophil activity. BF will not support the phagocytosis ofPseudomonasby normal neutrophils, and a subset of the BF samples suppressed the maximal mitogen response of control lymphocytes by more than 50%. The current work was carried out to analyze in depth the composition of BF using crossed immunoelectrophoresis. Twenty BF and six serum specimens collected from 20 patients between 6 and 18 hours postburn had 19 separate proteins measured. These proteins included immunologic proteins, antiproteases, acute-phase reactants, carrier proteins, and lipoproteins. A total of 546 protein measurements were made. The concentrations of all subgroups of proteins were significantly lower in the BF and burn serum specimens than in control serum. When the blister fluids were stratified according to their effect on normal lymphocyte activity, the suppressive blister fluid samples had higher levels of C3 and lower levels of I α I than the nonsuppressive samples. The elevation in C3 was secondary to the local activation of C3 and the generation of multiple C3 breakdown products. These changes in C3 are of potential biologic importance, since evidence has accumulated indicating that the various fragments of C3 can modulate both neutrophil and lymphocyte function. Thus, the results of this study suggest that local changes in the blister fluid may adversely affect local immunity and predispose the patient to burn wound sepsis.