We investigated the presence of a platelet-activating factor-like substance in blister fluid obtained from patients with bullous pemphigoid. Human platelet aggregation activity was present in blister fluids obtained from 4 out of 6 patients with bullous pemphigoid and in blister fluids obtained from 3 cases with contact dermatitis. Platelet aggregation activity of pemphigoid blister fluids was inhibited by pretreatment with lyso-PAF, which is a precursor metabolite of platelet activating factor. The activity of blister fluids obtained from patients suffering from contact dermatitis was not inhibited by precursor. Blister fluids obtained from burn lesions and blisters of normal skin induced in the suction revealed no platelet aggregation activity. These results suggest that platelet-activating factor or a similar substance is present in the blister fluid obtained from suffered bullous pemphigoid.