Leukemic leukocytes revealed 2 species of DNA polymerases: DNA-dependent and RNA-dependent polymerase activities. The template characteristics of the RNA-instructed enzymes were similar to those found with the DNA polymerase of oncogenic RNA viruses. Some minor differences in the molecular weight of RNA-instructed DNA polymerase activities could be taken as arguments for some heterogeneity of reverse transcriptase in different forms of leukemia. The most striking difference of these results is to the DNA polymerase content of normal leukocytes or blast cells, inasmuch as normal blood cells lack RNA-dependent DNA polymerase activities. The possible role of reverse transcriptase in transforming cells led to the search for inhibitors. Therefore, some rifamycins and distamycins have been tested for the influence on DNA-instructed DNA-synthesis in a cell-free system