A child with chronic myelocytic leukemia (CML), Philadelphia chromosome positive, developed a non-T cell, non-B cell, acute lymphocytic leukemia (ALL) during her blast cell crisis. The diagnosis was suggested by light microscopy and supported by histochemical stains and transmission electron microscopy. Immunologic studies showed the presence of a non-T, non-B leukemic blast population--indistinguishable from the most common form of ALL (null cell type). Markedly elevated terminal deoxynucleotidyl transferase (TdT) activity was found. The findings support the hypothesis that the primary cell involved in CML is a stem cell with pluripotential characteristics; frequently the blast cell proliferative phase terminates in acute myeloblastic leukemia, but it may also terminate in ALL. The TdT activity may be evidence of leukemic transformation and not necessarily related to the thymic origin of the lymphocytes.