Addition of human plasminogen to three different pemphigus plasma samples showed a synergistic effect on acantholysis in the skin organ culture model. Human plasmin itself, without addition of pemphigus plasma, induced typical acantholytic changes in the skin explants, causing different types of acantholysis in a dose- and time-dependent manner: in the presence of 3 CU plasmin per ml culture medium, focal suprabasilar acantholysis of pemphigus vulgaris type could be detected after 72 h incubation, whereas 15 CU/ml caused extended acantholysis of pemphigus foliaceus type in the upper epidermal layers after 24 h, and extended acantholysis of benign chronic pemphigus (Hailey-Hailey disease) type comprising all layers of the epidermis after 48 h incubation. Plasminogen activator levels (Mr 55,000 urokinase type) in tissue extracts of skin explants and in culture media were reduced after 24 and 48 h incubation with pemphigus IgG as compared to control experiments with normal human IgG; this probably resulted from urokinase inactivation by reaction with inhibitors. These results lend support to the hypothesis proposed by Hashimoto et al. in 1983 that the plasminogen activator-plasmin system could play an essential role in the protease mechanisms of pemphigus acantholysis.