Adjuvant activity of heat-killed Legionella pneumophila was demonstrated and compared with that of inactivated Mycobacterium tuberculosis H37Rv. The two species of bacteria were suspended separately in oil and Arlacel A. Bovine serum albumin (BSA) in saline was then emulsified within the respective adjuvants and injected intradermally into guinea pigs. Antibodies to the BSA antigen in the sera of the animals were quantitated with the kinetic-dependent enzyme-linked immunosorbent assay (k-Elisa). Guinea pigs immunized with BSA in adjuvant with killed L. pneumophila produced high titers of anti-BSA antibody, which, on the average, were nearly as high as in those immunized with BSA in complete Freund's adjuvant with M. tuberculosis H37Rv, and which were much greater than in others immunized with incomplete adjuvant, lacking bacteria. Moreover, with a polypeptide hapten, the L. pneumophila evoked as much or more antibody in rabbits as the mycobacterium adjuvant. The effect of the legionella adjuvant upon the cellular immune response was examined using skin tests. For this purpose guinea pigs were immunized with picryl-guinea pig albumin in these adjuvants. 6 weeks later, they were skin-tested with that antigen. They showed reactions which appeared to have immediate as well as delayed components when examined grossly and histologically. Others, immunized with incomplete adjuvant, did not exhibit delayed reactions. Accordingly, heat-killed L. pneumophila acts as a potent adjuvant. Under the circumstances of these experiments, it was as effective as heat-killed M. tuberculosis.