Because of their lympholytic action, glucocorticoids are included in most therapeutic regimens for the treatment of lymphomas and leukemias. The presence of functional glucocorticoid receptor may be predictive of the response to hormonal therapy in these diseases. We have developed an immunocytochemical procedure for human glucocorticoid receptor (GR) in order to assess its level and subcellular distribution in a well-studied system of childhood lymphoblastic leukemia cells (CEM), where sensitive and resistant subclones have been established. Several fixation and cell permeabilization protocols were compared. The most sensitive and reproducible one for light microscopy was prefixation in Bouin's solution followed by cytocentrifugation. Using various polyclonal and monoclonal antibodies, the GR was consistently localized predominantly in the cell cytoplasm in the absence of steroid. We compared localization of GR following glucocorticoid treatment in the glucocorticoid-sensitive clone CEM C7 with resistant subclones (4R4, 3R43, ICR27). Upon incubation with glucocorticoid, an increase in nuclear staining was clearly observed in the steroid-sensitive C7 cells. Although the resistant cell lines contain immunoreactive GR, they failed to show nuclear translocation following glucocorticoid treatment. This outlines the importance of the immunocytochemical procedure to distinguish between sensitive and resistant leukemic cells. Whether this test could be used prospectively to help select glucocorticoid therapy in human leukemias and lymphomas can now be examined.