Samples of serum and plasma obtained from 8 patients with acute pancreatitis were examined for the presence of complement-derived chemotactic activity for human PMN [polymorphonuclear leukocyte]. Significant chemotactic activity was found in acute phase serum and plasma samples from 5 patients. The presence of chemotactic activity was associated with reduced levels of CH50 [total hemolytic complement activity] as well as degradation products of C5 [complement component-5] (detected by a new method, i.e., radioimmunoelectrophoresis). The chemotactic activity was heat-stable (56.degree. C for 30 min), inhibitable by treatment with antibodies to human C5, and exhibited an apparent MW of 16,000 [daltons] (determined by chromatography on Sephadex G-75). These properties are identical with those of chemotactic C5-derived peptides (C5a and/or C5a des Arg). C5-derived peptides have been implicated as being mediators of acute lung injury (i.e., shock lung) in some clinical situations. 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.