Glucocorticoid (GC) receptor and terminal deoxynucleotidyl transferase (TdT) activities were studied in leukemia cells to investigate their diagnostic and therapeutic implications. Among cell lines with T-cell character, higher GC-receptor and TdT activities were found in T-ALL (HPB-ALL and ALL-Ichikawa) than in cells from adult pleomorphic T-cell leukemia (HPB-MLT). HPB-Null with pre-B cell-character exhibited moderate GC receptor but low TdT activity; Raji cells and CCRF-SB, derived from B-cell Burkitt lymphoma and B-ALL, respectively, manifested low GC receptor and no TdT activity. The highest GC receptor activity was demonstrated in null-cell ALL, followed, in order, by juvenile T-ALL, adult pleomorphic T-cell leukemia, and AML. Other kinds of lymphoid and monocytic leukemias exhibited low GC receptor and no TdT activity. Although low GC receptor and negative TdT were demonstrated in cells from seven out of nine patients under CML blastic crisis, the last patient had cells with positive TdT and GC receptor activity.