An assay for plasma free glucocorticoid receptor-binding activity has been developed with the use of specific glucocorticoid receptors in cultured rat pituitary tumor cells (GC line). Unlike other assays for corticosteroids, this assay measures the summed free (rather than the total) receptor-binding activity of all natural and synthetic steroids present in the sample. Plasma samples (0.5 ml) were incubated with [3H]dexamethasone and GC cells at 37 C for 45 min, and the nuclear bound dexamethasone was then measured. The free glucocorticoid receptor-binding activity was determined by the extent to which steroids present in the sample inhibit binding of the 3H-labeled steroids. Free cortisol concentrations of 0.3-3.6 micrograms/dl were accurately detected. That the assay measures free but not total steroids and that the free steroid provides the correct index of receptor-occupying capability were verified by two findings: 1) the assay yields similar results when the free cortisol is separated from plasma and then run in the assay, and 2) the receptor-binding activity of dexamethasone in plasma is reduced to the extent of plasma binding of this steroid measured directly. Free receptor-binding activity of adult plasma taken at various times determined by the assay correlated well with that expected from the total cortisol levels (measured by the corticosteroid-binding globulin-isotope assay) and the extent of plasma binding. The utility of the assay for measuring plasma dexamethasone levels after oral administration of the steroid is shown. The assay should be useful to determine the free glucocorticoid receptor-binding activity in plasma due to cortisol, synthetic glucocorticoids, or unidentified medication. In most circumstances, i.e. when antagonists are not present in sufficient quantities, such activity should equal the free glucocorticoid activity as well.