Complement-fixing intercellular antibodies were detected in 10 of 17 sera from untreated pemphigus patients. The role of complement in the organ culture system was investigated using these sera. Ten sera possessing complement-fixing intercellular antibodies showed IgG binding to the intercellular substance in the organ-cultured skin and acantholysis-like changes were observed in eight cases. C3 deposition was not seen in any case. However, after treatment of the sections of cultured skin with fresh normal human serum, complement fixation of the intercellular substance by bound IgG was revealed in all the ten cases. No significant differences in the grade of acantholysis-like changes between the complement-depleted system and the complement-supplied system were observed. Complement does not appear to be necessary in the acantholytic process in the in vitro organ culture system, even though we considered the presence of complement-fixing intercellular antibodies.