In view of the recent demonstration of complement consumption in human empyema, we searched for evidence of complement activation in 4 patients presenting initially with sterile postpneumonic effusions, but who were later shown to have contiguous loculated pleural empyemas. These results were compared with those obtained on 13 patients with postpneumonic uncomplicated effusions, and on 13 patients with primary empyema. C3d and Ba, respectively, breakdown products of complement components C3 and Factor B were both significantly elevated in the postpneumonic effusions of the 4 patients with loculated empyemas, when compared with those obtained in postpneumonic uncomplicated effusions (p less than 0.005). Total hemolytic complement activity (CH50) was similar in these 2 groups of pleural effusions. Plasma levels of C3d and Ba were undetectable, and CH50 values were within the same range in all groups of patients. These results suggest that the low molecular weight breakdown products of C3 and Factor B had diffused from the adjacent empyema, where active complement breakdown was occurring, into the sterile postpneumonic effusion. This study provides information about complement catabolism in extravascular sites during infection. The potential usefulness of this approach for the detection of loculated purulent collections is discussed.