The synthetic radioligand [3H]-R1881 binds to both androgen and progestogen receptors; these two types of receptor activity can be separated by competition experiments with radioinert steroids of defined biological activity. Using two standard tissues, rat prostate and human uterus, which are sources of androgen and progestogen receptors respectively, the optimal conditions for the determination of each type of activity were established. For the purposes of routine assay, androgen receptors were quantified after saturation of progestogen receptor sites with 125 nM radioinert R5020 using [3H]-R1881 and increasing concentrations of radioinert R1881. Progestogen receptor activity could be identified using the same radioligand and competition with radioinert progesterone or R5020, though for routine purposes, progestogen receptors were quantified using the more specific radioligand, [3H]-R5020. The binding of [3H]-R1881 to tumour cytosol was examined in 122 human breast cancers. Seventy-two tumours (59%) showed binding. Androgen receptor activity alone was present in 16 tumours, progestogen receptor activity alone in 30 tumours and both types in 26 tumours. Tumours containing progestogen receptor activity also showed binding to the progestogen [3H]-R5020, whilst those containing androgen receptors alone did not. Androgen receptor concentration varied from 17 to 210 fmol binding sites/mg cytosol protein (mean value 68) and the mean Kd was 2.15 X 10(-9) M. Progestogen receptor concentration varied from 25 to 1350 fmol binding sites/mg cytosol protein (mean value 410) and the mean Kd was 1.35 X 10(-9) M. The biological significance of the presence of these types of receptor in human breast cancers is currently being assessed from clinical follow-up.