In continuing search for exploitable biochemical differences between cancer and normal cells at the level of DNA replication, leukemic and "normal" hematopoietic cells from four different, established human cell lines were grown in culture flasks, and both the DNA and the DNA polymerase alpha were isolated in each case from the harvested (5-10 g wet weight) cell pellets. The four selected cell lines included a "normal" lymphoblastoid B-cell line (RPMI-1788), a pre-B cell (NALM-6) and a T-cell (MOLT-4) acute lymphoblastic leukemias, and a promyelocytic leukemia (HL-60). The DNA polymerase alpha enzyme of the two B-cell lines (both the leukemic and the "normal") showed the usual sensitivity toward inhibition by aphidicolin, while those from the two other leukemic cell lines were remarkably resistant to the antibiotic. Partially thiolated polycytidylic acid (MPC) strongly inhibited only the DNA polymerase alpha of the "normal" cell line, whereas the corresponding enzymes of all three leukemic cell lines were relatively insensitive to MPC. In contrast, the partially thiolated DNAs derived from the leukemic cell lines more strongly inhibited the DNA polymerase alphas of the leukemic cell lines than that of the "normal" cell line. These results indicate the existence of some structural differences between the DNA polymerase alpha enzymes (as well as between the DNAs) of human cells of different lineage and, particularly, of leukemic vs. "normal" character; such differences could be exploited in the design of selective antitemplates for chemotherapy.