An experimental feeding trial was conducted in order to substantiate an hypothesis on the aetiology of a previously unrecorded cardiomyopathic syndrome in beef cattle in Israel. It was believed that residues of the ionophore maduramicin (Cygro; American Cyanamid) in poultry litter fed to cattle, after the maduramicin had been incorporated into broiler feed as a coccidiostat, were the cause of the cardiomyopathy. Three groups of 5 heifers were fed for several weeks (1) poultry litter from a field case, containing 4.8 ppm maduramicin; (2) poultry litter to which maduramicin was added to give a concentration of 12 ppm; (3) poultry litter with no maduramicin (control). Clinical, biochemical, necropsy and histopathological findings showed that maduramicin residues are cardiotoxic, even at the lower level of maduramicin fed, which is commonly encountered in poultry litter in Israel.