Samples of serum and plasma obtained from eight patients with acute pancreatitis were examined for the presence of complement-derived chemotactic activity for human PMN. Significant chemotactic activity was found in acute phase serum and plasma samples from five patients. The presence of chemotactic activity was associated with reduced levels of CH50 as well as degradation products of C5 (detected by a new method, i.e., radioimmunoelectrophoresis). The chemotactic activity was heat-stable (56 degrees C for 30 min), inhibitable by treatment with antibodies to human C5, and exhibited an apparent molecular weight of 16,000 (determined by chromatography on Sephadex G-75). These properties are identical with those of chemotactic C5-derived peptides (C5a and/or C5a des Arg). Recently, C5-derived peptides have been implicated as being mediators of acute lung injury (i.e., "shock lung") in some clinical situations. It is intriguing to speculate, therefore, that circulating C5-derived chemotactic peptides may play a role in the pathogenesis of the lung injury observed in some patients during the course of acute pancreatitis.