By means of the radial immunodiffusion-assay technique the concentration of serum complement C′3 (β1C — β1A - globulin) in 18 patients with pemphigus and pemphigoid was determined. It was normal in 11, elevated in 6, and slightly depressed in 1 patient. Of seven patients with severe, widespread, untreated disease, three had increased serum C′3 levels, and one had a slightly depressed level. The sera of four patients were tested before and during treatment and the changes in serum C′3 levels were unrelated to the treatment or to the severity of the disease. These findings do not exclude the pathogenic involvement of complement in pemphigus and pemphigoid since (1) consumption of C′3 would have to reach drastic amounts to be detected or (2) other complement components may be essentially involved in the process of acantholysis.