Cleavage of the fifth component of complement yields C5a, a potent neutrophil (PMN) and eosinophil chemoattractant, and modulator of microvascular permeability. Similarly, but to a lesser degree, C3 increases vascular permeability and histamine release. SC-41930 (7-[3-(4-acetyl-3-methoxy-2-propylphenoxy)-propoxy]-3,4-dihydro-8- propyl- 2H-1-benzopyran-2-carboxylic acid), an orally-active antiinflammatory agent was tested in an in vivo model of dermal PMN chemotaxis induced by r-hu-C5a and hu-C3. Intradermal injection of C5a in the guinea pig resulted in a significant dose-dependent influx of PMNs at 4 hours as assessed by the dermal levels of myeloperoxidase (MPO). SC-41930 (20 mg/kg) given orally to guinea pigs with intradermal injections of 1 microgram C5a significantly (p less than 0.001) reduced dermal MPO content. SC-41930 was less potent against C3, requiring 40 mg/kg to significantly reduce dermal MPO levels. Agents such as SC-41930, which nullify complement's proinflammatory properties, may well have therapeutic potential.