The action of certain dipeptides (Lα-alanyl-histidine, Lβ-alanyl-histidine, histidyl-leucine, and glycyl-L-histidine) and of histidine itself on passive cutaneous anaphylaxis was studied in guinea pigs. Lβ-alanylhistidine (carnosine) and Lα-alanyl-histidine were found to inhibit passive cutaneous anaphylaxis. Histidyl-leucine, glycyl-histidine, and histidine had no inhibitory action on passive cutaneous anaphyla