Binding of 3H-cortisol (3H-C) and 3H-dexamethasone (3H-DX), a fluorinated glucocorticoid which does not bind to Corticosteroid Binding Globulin (CBG), was investigated in two different target tissues, human breast cancer and human leukemic lymphocytes, using a modified DCC assay method. Maximal binding to cytosol occurred, when increasing concentrations of 3H-C or 3H-DX were incubated in the presence or in the absence of an excess of cold cortisol or dexamethasone, at 0 degrees C for 1 hr or 4 hr, respectively. A 50 sec exposure to DCC was sufficient to separate bound from free labelled steroid. Data presented suggest that two different classes of glucocorticoid exist, which are differently distributed in target tissues, and may be identified on the basis of affinity for the tracer, binding specificity and thermolability.