Macrophages from peritoneal exudate contain three types of DNA dependent RNA polymerase. The activity of RNA polymerases in macrophages derived from normal animals is very low. Guinea-pigs were immunized by sheep red blood cells. The immunization enhanced the activity of the RNA polymerase of macrophages derived from peritoneal exudate. The RNA polymerase activity was tested after the solubilization and chromatographic resolution of the three types of polymerases with exogenous template. The results obtained indicated that the immunization enhances the levels of polymerase I and III 10 fold while the level of polymerase II increased 5 fold.