Culture supernatants of Legionella pneumophila, Philadelphia 1, were found to have proteolytic activity, as well as a nondialyzable, heat‐labile cytotoxin for Chinese hamster ovary (CHO) cells, and a factor which caused hemorrhagic dermal ulceration when injected intradermally into mice. A protease was purified from culture supernates by filtration on Sephacryl S‐200 followed by chromatography on DEAE cellulose. Proteolytic activity had a pH optimum of 5.5, and migrated as two bands in PAGE, with molecular weights of 42 and 31 kDa. CHO cell cytotoxic, dermal ulcerative, and proteolytic activities copurified. The results are consistent with the same protein being responsible for these activities.