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Evidence is presented that DNA polymerases from human leukemic cells are mutagenic. Nucleic acid-free extracts of acute lymphoblastic leukemic cells polymerized about 10-times more dCTP using poly-(dA-dT) * poly(dA-dT) as a template than did extracts from normal lymphocytes. Mutagenic DNA polymerases could perhaps play an important role in tumor progression.We have started to inquire whether DNA synthesis in malignant cells is less exact than in normal cells. Some of the characteristics of tumor progression (1), namely, (a) continuing evolution of new cell variants, (b) uncontrolled cellular proliferation, and (c) increasing number of chromosomal aberrations, suggest that as malignant cells become progressively anaplastic there is an accumulation of errors in genetic information.For study of the fidelity of in vitro DNA synthesis, human lymphocytes offer several advantages. Leukemic lymphocytes of the acute "blastic" type have considerable DNA polymerase activity. DNA polymerase activity in normal lymphocytes can be increased 30-to 200-fold by stimulating them with phytohemagglutinin (2). This increase parallels the ability of these lymphocytes to incorporate thymidine into DNA, suggesting that the polymerase measured in vitro functions in chromosomal replication. Thus, there is sufficient polymerase activity in normal stimulated lymphocytes and their malignant counterparts to analyze and compare the exactness of DNA synthesis. The exceptionally low deoxyribonuclease activity in these cells facilitates meaningful experiments with partially purified enzymes (2). We have chosen to investigate the fidelity of DNA synthesis using peripheral blood lymphocytes rather than cells from longterm tissue culture in order to relate our findings more directly to the biology of the disease state. In these initial experiments we measured the ability of "nucleic acid-free extracts" of stimulated normal and leukemic lymphocytes to exactly copy the base sequences of the synthetic homopolymers poly-(dA-dT) -poly(dA-dT) and poly(dG) * poly(dC). METHODSEnzymes. Lymphocytes from normal donors and from patients with acute lymphatic leukemia were purified as described (2). Leukemic lymphocytes were obtained from untreated patients with white blood cell counts greater than 40,000 cells per mm2. Even though acute leukemic lymphocytes are probably not stimulated by phytohemagglutinin, both the normal and leukemic cells were cultured with phytohemagglutinin for 3 days (3). Cell-free extracts were obtained by disruption of the cells with repeated freeze-thawing, elimination of DNA by phase extraction (4,5) DNA Polymerase Activity. To measure the fidelity with which poly(dA-dT) * poly(dA-dT) is copied in lymphocyte extracts, we measured the incorporation of correct nucleotides (dATP and dTTP) and incorrect nucleotides (dGTP or dCTP) in separate assays using the same enzyme preparations. For determination of total synthesis, a standard reaction mixture was used containing in a total volume of 0.2 ml: 13.0MM [3H ]dTTP (1500 dpm/pmol), 1...

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