Previously, we reported (1967, Proc. Natl. Acad. Sci. 57, 673) that new species of RNA appeared in the spleens of mice after the injection of antigen. These species were detected by molecular hybridizations between complementary strands of RNA and DNA. We used the membrane filter method (D. Gillespie and S. Spiegelman, 1965, J. Mol. Biol. 12, 829).