A fast, reliable and sensitive radioimmunoassay method using either 3H-hexoestrol or a 125I-hexoestrol derivative as the competitive radioligand has been developed for the measurement of residues of hexoestrol in faeces. The blank value for untreated cattle was 113 +/- 98 pg/g faeces, the intra-assay coefficient of variation 10.8 per cent, interassay coefficient of variation 11.1 per cent and recovery of 3H-hexoestrol added to faeces was 58.7 per cent. The concentration of hexoestrol in faeces was measured in 18 steers and 14 bulls at regular intervals after implantation of 36, 45 and 60 mg of hexoestrol. Residues were detected in all samples up to 104 days after implantation and in nine out of 13 samples taken 111 to 153 days after implantation. Residues of hexoestrol were also measured in edible tissues and body fluids of 14 bulls and eight steers slaughtered between 90 and 153 days after implantation of 45 mg hexoestrol. The percentages of samples containing residues were 82, 73, 64, 82 and 27 per cent for bile, urine, liver, kidney and muscle, respectively.