Human DNA ligase was purified from different kinds of immunocompetent cells: thymocytes, normal and stimulated lymphocytes, blasts from ALL (Burkitt and non-T, non-B) and ANLL (M1, M2, and M5). Based upon the protocol for the treatment of these leukemias, the purified enzymes were assayed in the presence of routinely used combinations of antileukemic drugs. At the range of concentration tested (between 0.1 and 5 microM) some drugs taken separately were totally inactive on the enzyme from the different sources. For those being inhibitory, when used in combination their effect was always different from what was observed when the compound was tested alone. Some combinations were more effective in inhibiting the enzyme from leukemic than from normal cells (vincristine + cyclophosphamide + prednisone in ALL and rubidazone + Ara-C, Ara-C + m-AMSA, in ANLL). However, some combinations of drugs are without effect on ligase from leukemic cells at this dose range (vincristine + rubidazone + Ara-C + prednisone and adriamycin + asparaginase + Ara-C in ALL or etoposide + Ara-C, adriamycin + cyclophosphamide in ANLL). This is the first direct observation of the effect of cytostatic drugs on DNA ligase, a key enzyme of the DNA replication and repair process. The clinical consequences of these observations are discussed in an attempt to selectively inhibit replication, thereby division, of cancer cells.