Rapid solvent extraction combined with a radioimmunoassay using a nionoclonal antibody raised against a derivative of zeranol has been used to measure the residues of the anabolic agent zeranol in the edible tissues (muscle, liver, kidney and fat) of cattle treated with Ralgro.Calibration curves, both with and without, tissue extracts exhibit good parallelism. Regression analysis for the extraction of‐ zeranol from tissues dosed with standard amounts of zeranol have correlation coefficients of 0.979, 0.99 1, 0.986 and 0.985 for muscle, liver, kidney and fat, respectively.The limits of decision defined as the mean value + 3 SD for the concentrations apparently observed (noise) in tissues from animals not treated with Kalgro were 278, 121, 373 and 110 ng/kg for muscle, fat, liver and kidney, respectively. In the tissues of 4 cows implanted with Ralgro (36 mg), and sampled 70 clays after iniplanting, the highest concentration of zeranol in each tissue was 232 ng/kg (muscle), 391 ng/kg (liver), 287 nglkg (kidney) and 293 nglkg (fat), and residues were detected in all samples of fat (4), 3 kidney samples and I liver sampl