Purine riboside (nebularine, 9-beta-ribofuranosylpurine) is a naturally occurring base analog which closely resembles adenosine. It inhibits carcinogenic growth. Purine riboside strongly inhibits RNA and DNA synthesis in different cancer ascites cells. Gel electrophoretic analysis of RNA synthesis in vivo in the presence of purine riboside shows the ribosomal components to be inhibited the most. A method for assaying purine riboside or its phosphates intracellularly has been devised, and by using this it has been shown that purine riboside is extensively phosphorylated in the cells. The triphosphate derivative of purine riboside has been isolated and tested in the Escherichia coli RNA polymerase assay. It appears not to be incorporated into this type of RNA and to competitively inhibit this reaction with regard to ATP.