We investigated the pathologic, bacteriologic and immunologic responses of BALB/c-nu/nu mice (nude mice) and BALB/c mice (euthymic mice) infected intravenously with virulent and avirulent Rhodococcus equi ATCC 33701, and its plasmid-cured derivative ATCC 33701P(-), to evaluate the role of T lymphocytes. Adaptive transfer of immune and normal spleen cells into nude mice was also investigated. Nude and euthymic mice were inoculated with 10(6) ATCC 33701 or 10(6) ATCC 33701P(-) intravenously (i.v.) and killed at 0, 7, 14, 21, 28 and 35 days post-inoculation, except dead cases. In athymic nude mice infected with ATCC 33701, deteriorating systemic inflammatory responses developed during the experimental period and multiplication of the bacteria continued until the end of the experiment. Nude mice developed splenomegaly and multifocal gross hepatic necrosis with some mortality. Splenomegaly was caused by diffuse proliferation of bacteria-laden macrophages and epithelioid cells, and gross hepatic necrosis was caused by the formation of thromboses and granulomatous lesions. Infection of euthymic mice with a sublethal dose of ATCC 33701 resulted in transient granuloma formation in the liver and spleen, production of specific antibodies against the virulent bacteria and gradual elimination thereof. In contrast, infection with ATCC 33701P(-) produced few lesions after rapid elimination and no antibody production against bacteria in either normal or athymic nude mice. In nude mice given normal and immune spleen cells, histopathological lesions and granulomas formed only in the Liver and spleen, in addition to specific antibodies against 15- to 17-kDa antigens. The pathological lesions observed in the nude mice given immune spleen cells were similar to those seen in the mice given normal spleen cells, but they were less severe than those in mice given normal spleen cells. Mice given immune spleen cells showed a significantly higher elevation of antibody production than mice given normal spleen cells. These results suggested that protection against virulent R equi in mice depends mainly on cell-mediated immune responses, whereas avirulent R equi in mice are cleared by innate immune responses.