Limited proteolysis of the glucocorticoid receptor has proven to be a valuable tool for a functional analysis of the receptor protein. With the help of these analyses, it has been possible to describe three functional domains of the receptor protein. The native glucocorticoid-receptor complex contains a steroid-binding domain (A), a DNA-binding domain (B) and an immunoactive domain (C). This form of the glucocorticoid receptor has a Stokes radius of 6.1 nm and a molecular weight of 94 K when purified. Two steroid-binding proteolytic receptor fragments can be found. The larger one has a Stokes radius of 3.3 - 3.6 nm and a molecular weight of 39 K and contains both the steroid- and DNA-binding sites (A + B). The smaller steroid-binding receptor fragment, with a Stokes radius of 1.9 nm and a molecular weight of 27 K, contains only the steroid-binding domain (A). Analysis of the proteolytic fragments of the glucocorticoid receptor using the specific anti-receptor antibodies revealed the occurrence of a fragment with Stokes radius 2.6 nm following limited proteolysis of the receptor by alpha-chymotrypsin. This fragment contains neither the steroid-binding nor the DNA-binding domains but consists only of the immunoactive domain (C). Further proteolysis of this fragment results in an even smaller form with Stokes radius 1.4 nm. The apparent identity of the larger of the two proteolytic forms of the glucocorticoid receptor (the 3.3 - 3.6 nm form) with the receptor isolated from certain corticosteroid-resistant cells, together with the lack of the immunoreactive domain in these cells appears to indicate an important function of this domain with regard to the biological activity of the receptor.