The DNA-dependent RNA polymerase of nuclei from lymphoid-rich and erythroid-rich mouse spleens was compared in regard to requirements and conditions of the reactions. The inhibition by either actinomycin D or pancreatic DNase indicated that a DNA template was required for the observed reactions. Following the induction of a hemolytic anemia by the administration of phenylhydrazine, there was a tenfold increase in the nuclear polymerase activity per milligram nuclear DNA of the developing erythropoietic spleen when the assays were done at low ionic strength and more than a threefold increase at high ionic strength. The peak rise in polymerase activity precedes the maximal development of the erythropoietic spleen by 3 days.

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