Suspensions of rat mast cells were used to study the histamine-releasing actions of anaphylatoxins C3A and C5a in vitro. The peptides, derived from human or porcine complement proteins C3 and C5, were less potent than 48/80 but more potent than bradykinin in stimulating release of histamine from mast cells. The pattern of release resembled that of the anaphylactic release action, e.g. release was limited to less than 30 per cent of the cell histamine, the reaction was calcium-dependent and was potentiated by phosphatidyl serine. When C3a and C5a were added together to mast cell suspensions, the amount of histamine released was additive. Similarly, release by either peptide combined with bradykinin was additive. Histamine-releasing activity (as well as smooth muscle-stimulating activity) was abolished when the peptides were treated with pancreatic carboxy-peptidase B. Active or inactive peptides were bound by mast cells and addition of active C3a in combination with the inactive, des-arginine derivative, C3ai, resulted in partial inhibition of histamine release.