The recently discovered DNA polymerase iota differs greatly from the numerous eukaryotic and prokaryotic DNA polymerases known previously in its ability to catalyze error-prone DNA synthesis. Using homogeneous preparations of the enzyme, it was shown previously that DNA polymerase iota incorporated preferentially dGMP opposite the thymidine of the template in the growing DNA chain. To elucidate the role of this enzyme in the mammals, its activity was assayed in crude cell extracts of different mouse organs. It is shown that the extracts of the brain and testis cells exhibit the highest activity of DNA polymerase iota, which is not in agreement with the results of other authors. The data suggest that the tissue specific expression of DNA polymerase iota is regulated to a significant degree at the posttranscriptional and posttranslational levels.