Two DNA polymerases that can copy synthetic RNA polymers are present in human tissue culture cells. These enzymes which have each been purified about 500-fold, are present in both HeLa cells, which are derived from a cervical carcinoma, and in WI-38 cells, a normal diploid strain originating from human embryonic lung tissue. These synthetic RNA-dependent DNA polymerases are identified by their ability to copy efficiently the ribo strand of synthetic oligonucleotide-homopolymer complexes, and differ in this respect from the known DNA-dependent DNA polymerases found in HeLa cells. The template requirements of these new DNA polymerases resemble that of the RNA-dependent DNA polymerases of the RNA tumor-viruses.