Rifampicin quinone is an immunosuppressant, but not rifampicin itself.

Rifampicin as a potential immunosuppressive agent was tested in the rat. A freshly made-up solution of this labile antibiotic in a daily dosage of 20 mg/kg iv did not affect mean graft survival time in a heterotopic heart transplant model. However, when stored solutions of rifampicin were used mean graft survival time was significantly prolonged (from 8.2 +/- 0.4 days with the controls to 18.1 +/- 1.2 days). A similar prolongation was observed when rifampicin quinone, the major rifampicin oxidation product, was administered. We conclude that the immunosuppressive effect ascribed to rifampicin is in fact due to its oxidation product, rifampicin quinone.