Free and conjugated zeranol residues determined by radio-immunoassay in urine and plasma of calves treated with Forplix.

Anti-zeranol 7-hemisuccinate-bovine serum albumin was raised in rabbits. This antiserum was used in a radio-immunoassay of zeranol residues in urine and plasma of calves implanted with Forplix. The antibody was specific for zeranol but cross-reacted with its metabolite zearalenone and the mycotoxin zearalenone. Detection limits in plasma and in urine were 100 pg/ml and 1 ng/ml respectively. In veal calves treated with zeranol containing implant, no residues were detected in plasma even if plasma proteins were hydrolysed with pronase before the radio-immunoassay. Free and conjugated residues in urine were easily measured. The urine concentration of conjugated residues increased markedly after the 20th day of treatment and was still high (19 ppb) at day 40.