Haemolytically inactive C5b67 (iC5b67), which was made from purified human components and decayed to a haemolytically inactive form, was evaluated as an agonist for murine leucocytes both in vitro and in vivo. In an in vitro assay, iC5b67 stimulated chemotaxis for both neutrophils purified from mouse bone marrow and splenic eosinophils of IL-5 transgenic mice. The stimulation was dose-dependent, with high dose inhibition. As with human neutrophils, iC5b67 also failed to up-regulate CR3 (CD11b/CD18) expression and to stimulate superoxide generation in murine bone marrow neutrophils, in vitro. In vivo, iC5b67 elicited an inflammatory response in a mouse model of pleuritis. A marked infiltration of neutrophils, which peaked at 4 h, was followed by an infiltration of eosinophils and mononuclear leucocytes. This inflammatory response was dose- and time-dependent. However, the protein concentration in the pleural wash fluid did not increase, indicating that iC5b67 did not induce a capillary leak. Although the infiltration of neutrophils could not be reproduced by pure C7 or human serum albumin (HSA), C5b6 did induce an influx of neutrophils. We were able to document the existence of C7, both antigenically and functionally, in pleural washes of normal mice, making it likely that the activity of C5b6 resulted from the in situ formation of C5b67 and iC5b67. The mouse model of pleuritis promises to be a useful in vivo system in which to evaluate the pro- and anti-inflammatory effects of iC5b67 that have been noted in vitro.