Particulate DNA polymerase activity that copied poly(2'-O-methylcytidylate) . oligodeoxyguanylate and banded at a density of 1.15 to 1.20 g/ml in sucrose gradients was detected in 8 of 16 human ovary tumors and in 11 of 16 malignant prostate tissues. None of the 10 nonmalignant ovary and prostate tissues examined contained detectable particulate DNA polymerase activity that copied poly(2'-O-methylcytidylate) . oligodeoxyguanylate. Since poly(2'-O-methylcytidylate) . oligodeoxyguanylate is effectively copied by oncornavirus RNA-directed DNA polymerase (reverse transcriptase) although not by the known species of human cell DNA polymerase, these results are interpreted as supporting the concept that some malignant human tissues contain particle-associated reverse transcriptase activity.