Inflammatory exudates from 10 patients with bullous skin diseases were analysed by immunochemical techniques including crossed immunoelectrophoresis. The results were compared with those obtained in fluid from suction bullae obtained in normal skin in 13 control subjects and synovial fluid from 20 patients with rheumatoid arthritis. Abnormal fibrinogen degradation products identical with those found in synovial fluid from patients with rheumatoid arthritis were detected in exudates from each of the patients with bullous dermatoses, whereas significantly smaller amounts of fibrinogen antigenic material were detected in fluid obtained by suction. The fibrinogen antigenic material demonstrated in exudates from pathological bullae, immunochemically similar to that found in rheumatoid synovial fluid, indicates that the presence of these products reflects the more general features of an inflammatory exudate.