The rate of synthesis of RNA in the thymus glands of adult mice increased after immunization with sheep red blood cells (SRBC). The specific activity of some fractions of RNA, separated first by density gradient centrifugation and then by polyacrylamide gel electrophoresis, was 16-fold higher on day 3 after immunization than control mice not injected. RNA synthesis in the thymus was inhibited by rabbit anti-mouse thymus serum, injected along with antigen.

A material was found in RNA extracts from the thymus glands of mice immunized with SRBC which converted a small proportion of either spleen cells or peritoneal cells from nonimmunized mice to form sheep cell hemolysins. Neither extracts from the glands of nonimmunized mice nor the livers of immunized mice were active. Extracts from the thymus glands of mice immunized with rabbit red blood cells (RRBC) were inactive and activity was destroyed by ribonudease. The residual antigen content was not determined. Biologically active extracts from the thymus had a different electrophoretic mobility from active extracts from the spleen.

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