A series of twelve recently synthesized 6-substituted derivatives of 9-beta-d-ribofuranosylpurine 3\',5\'-cyclic phosphate (RPcMP) were evaluated for in vitro antiviral activity, using inhibition of viral cytopathogenic effect as the primary parameter for evaluation. Inhibition of the development of intra- and extracellular virus titer was used as a secondary criterion with certain viruses. Five derivatives were considered to have significant antiviral activity. 6-Hydroxylamino-RPcMP was active against type 1 herpes simplex, cytomegalo-, and vaccinia viruses. 6-Thio-RPcMP was inhibitory to types 1 and 2 herpes simplex, cytomegalo-, vaccinia, and type 3 parainfluenza viruses. The 6-methylthio derivative was active against types 1 and 2 herpes simplex, cytomegalo-, and vaccinia viruses, and types 1A, 2, 8, and 13 rhinoviruses; alteration of this 6-substitution to 6-ethylthio or to 6-benzylthio weakened the herpes- and vaccinia virus activity of the compound, but each continued to have significant antirhinovirus activity. The effect of time of addition of 6-methylthio-RPcMP to type 1 herpes simplex virus-infected cells was determined; the compound was most active when added prior to the virus. Early removal of the compound from the infected cells markedly reduced its antiviral effectiveness.