The interaction with the glucocorticoid receptor of RU 486, a recently described antiglucocorticoid, was investigated in intact cells. When incubated at 37 degrees C with intact rat thymocytes [3H] RU 486 underwent negligible nuclear transfer. Moreover when assayed in physiological buffers, i.e. physiological ionic strength and absence of molybdate, the cytosolic [3H] RU 486-receptor complexes obtained displayed a 7-8 nm Stokes radius after analysis by high performance size exclusion chromatography (HPSEC). These high size complexes appeared stable in the native cytosol but dissociated during sucrose gradient centrifugation. Western blot analysis of the fractions obtained after HPSEC separation was performed using a monoclonal antibody able to recognize the 90K non steroid binding protein associated with the molybdate stabilized glucocorticoid receptor complexes. This antibody clearly demonstrated the presence of a 90K non-steroid binding protein in the 7-8 nm peak obtained with [3H] RU 486 receptor complexes. On the contrary [3H] triamcinolone acetonide in the same conditions yielded a 5 nm peak of transformed receptor which did not contain the 90K protein. Thus RU 486, in absence of molybdate, stabilized the 90K protein-receptor interaction in intact cells, an event probably related to its antiglucocorticoid activity.