We determined the mol wt of glucocorticoid receptors in human leukemia cells in order to detect glucocorticoid receptor defects that might cause glucocorticoid resistance. Glucocorticoid receptors in intact cells were affinity labeled with [3H]dexamethasone-21-mesylate and were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Receptors in normal human peripheral blood mononuclear cells and six human leukemia cell lines had mol wt of 97,000. Malignant cells from ten of 25 patients with leukemia contained electrophoretically abnormal glucocorticoid receptors having mol wt of 55,000 in addition to normal size receptors (Mr = 97,000). The receptor abnormality was not restricted to a particular type of leukemia and was seen in cells from both newly diagnosed patients and patients who had received prior chemotherapy, including prednisone. The abnormal receptor was not generated when cells having only normal size receptors were assayed under conditions that favor proteolysis or when cytosol from cells containing the abnormal receptor form was mixed with cytosol from cells containing only normal size receptors. The mol wt of the abnormal receptors in human leukemia cells was the same as the mol wt of receptors in mutant mouse lymphoma cell lines, S49 143R and S49 55R, which have the nuclear transfer-increased phenotype of glucocorticoid resistance. This work describes for the first time a single common electrophoretic abnormality of glucocorticoid receptors in human leukemia cells. Further investigation of glucocorticoid receptor defects in human leukemia cells could lead to an improved understanding of the mechanisms of glucocorticoid resistance in leukemia as well as a method of predicting which patients are likely to be resistant to glucocorticoid therapy.