C 5a is an 11,000-D fragment of the fifth component of complement (C5) with potent anaphylatoxic and leukocyte chemotactic activities. C5a is believed to play an important role in the pathophysiology of certain skin disorders and systemic diseases with cutaneous manifestations. However, there is very little known about the in vivo reactivity of C5a in man. In this study we examined the effects of intradermal injections of human C5a in 17 normal volunteers. C5a was prepared by interacting highly purified human C5 with zymosan bound alternative pathway C5 convertase under conditions resulting in consumption of approximately 90% of the C5 substrate. C5a produced in this manner was chemotactic for human neutrophils and monocytes (0.5 X 10(-7) to 10(-9) M) and caused neutrophil aggregation and myeloperoxidase release (concentrations greater than or equal to 10(-10) M) in vitro. In vivo, C5a produced immediate wheal and flare reactions in all volunteers, and was active in doses as low as 1 ng (10(-13) mol). Intradermal testing with 20 ng of C5a in eight volunteers produced a maximal mean wheal of 11.75 mm (+/- 0.80 mm SEM) 20 min after anaphylatoxin injection, and a maximal mean erythema of 62.50 mm (+/- 3.27 mm SEM) 10 min after C5a administration. Reactions at C5a test sites were dose-related, associated with marked pruritus in some subjects, resolved without lesion formation, and were not associated with late phase reactions. In vivo testing revealed that human C5a was a more potent mediator of wheal and flare reactions than histamine, 48/80, human C3a, or morphine sulfate. Skin biopsies from eight volunteers 20 min after intradermal injection of 20 ng of C5a revealed a neutrophil-predominant perivascular infiltrate, endothelial cell edema, and sites of leukocytoclasis. Mast cell degranulation was observed on both light and electron microscopy of biopsies from C5a test sites. Although erythema at C5a injection sites was reduced by pretreating volunteers with hydroxyzine, whealing reactions and cellular infiltrates in biopsies were unaffected by this H1-antihistamine. Moderate doses of systemic corticosteroids did not alter clinical or histologic reactions at C5a injection sites in two volunteers. This study, using doses within the potential physiologic range of the anaphylatoxin, provides a comprehensive assessment of the effect of human C5a on normal human skin.