The primary beta-cell antigen of insulin-dependent diabetes is thought to be a protein with a molecular weight of approximately 64 kD. Hyperthermic incubation and cytokines such as interleukin 1 beta, gamma interferon, and tumour necrosis factor induce synthesis of 64 kD protein by insulinoma cells. By western blot techniques, cross-reactivity was found between this 64 kD protein and monoclonal antibodies directed against Mycobacterium tuberculosis heat-shock protein 65, but not with antibodies directed against a similar epitope of M leprae heat-shock protein 65. Binding of M tuberculosis heat-shock protein 65 antibodies to interleukin-1 beta-treated cells was inhibited by prior addition of serum from insulin-dependent diabetic patients which contained antibodies to 64 kD beta-cell antigen. It is suggested that heat-shock protein 65 may be the 64 kD beta-cell antigen and that autoreactivity to an epitope of heat-shock protein 65 may confer susceptibility to insulin-dependent diabetes mellitus.