We examined the incorporation of 6-thioguanine into the RNA of Sarcoma 180 to determine the relative distribution of this drug in the various RNA fractions. 6-Thioguanine-3 s S was recovered from ribosomal RNA, transfer RNA, and rapidly labeled RNA (probably messenger RNA) as the intact compound. The incorporation of the analog into microsomal RNA was inhibited by actinomycin D and was enhanced by azaserine. 6-Thioguanine caused an immediate decrease of uridine labeling of the cytoplasmic

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... mal RNA and transfer RNA; at the same time, protein biosynthesis was inhibited. Maximal inhibition was observed 3 to 4 hr after the administration of the base analog. No inhibition of uridine labeling of the rapidly labeled cytoplasmic RNA was evident until 3 hr after drug treatment. The possible role of macromolecular effects of thioguanine on growth inhibition is discussed.