The central features of Kawasaki disease are immune activation and cytokine-mediated generalized vasculitis. To identify the predisposing factors, we examined the antibody response to BCG antigens, since reactivation of a previous BCG inoculation site is an early, specific manifestation of this disease. BCG antigens were separated on SDS-PAGE, transferred to membrane, and incubated with acute- and convalescent-phase sera of 21 patients with Kawasaki disease. Sera were also examined for the presence of antibodies to mycobacterial 65-kDa heat-shock protein (HSP65), and to its human homolog P1 antigen using synthetic peptides of nonhomologous region. To demonstrate the HSP65-sensitized T cells, in vitro proliferation assay was performed. All convalescent, but not acute phase, sera showed a strong antibody reactivity against 65-kDa protein. The reactivity was directed to recombinant HSP65. Non-cross-reactive sequences between rHSP65 and human HSP65 cognate were synthesized. The sera recognized these peptides of rHSP65 and autologous P1 antigen. Peripheral lymphocytes proliferated following the addition of rHSP65 (stimulation indices, 2.16-7.82; mean, 4.54). These findings suggest that HSP65 may be the most potent factor predisposing to Kawasaki disease, and that an autoreactivity to the epitope of the human HSP65 homolog may be related to the susceptibility to the disease.