Isolated adrenocortical cells, prepared by collagenase disaggregation of normal rat adrenal glands, have been used to study the binding characteristics of 125I-labelled corticotrophin (ACTH) of established biological activity. The binding of the labelled hormone to these whole cells was highly specific, only peptides possessing steroidogenic activity displaced the labelled hormone. Binding was rapid, being complete within 5 min of adding the hormone, and the amount of hormone bound remained constant for up to 20 min thereafter. Over the range 160–10000 pg ACTH/ml, increased binding of the hormone was observed at all concentrations of hormone which stimulated steroidogenesis. However at levels of ACTH which stimulated maximal steroidogenesis there was no saturation of binding. This provides the first direct evidence for the existence of 'spare receptors' for ACTH on whole adrenocortical cells. Scatchard analysis of the binding data suggests that there are two types of receptor for ACTH in this preparation of cells. One receptor is of high affinity (dissociation constant = 2·5 × 10−10 mol/l) about 3000 sites/cell and the other is of lower affinity (dissociation constant = 1 × 10−8 mol/l) with about 30000 sites/cell.