Toxoplasmosis of both donor and recipient hearts was diagnosed by means of endomyocardial biopsy specimens after heterotopic cardiac transplantation for dilated cardiomyopathy. Before transplantation, the donor had raised antibody titers to Toxoplasma, and the recipient was negative. When toxoplasmosis was diagnosed on the basis of endomyocardial biopsy specimen, the recipient had a greatly elevated antibody titer of 1:1,027. This suggests that the infection could have been transferred with the donor heart. The mononuclear cell response elicited by disrupted toxoplasmic cysts interferes with the diagnosis of rejection in graft biopsy specimens. Electron microscopy is valuable in confirming a light microscopic diagnosis of toxoplasmosis. Drug therapy eradicated the toxoplasmosis, but the patient died later of tuberculous meningitis.