9-(beta-TD-xylofuranosyl)guanine (xylo-G) was converted chemically to the 9-(beta-D-xylofuranosyl)guanine 5'-monophosphate (xylo-GMP) and 9-(beta-D-xylofuranosyl)guanine cyclic 3',5'-monophosphate (c-xylo-GMP). These compounds were tested against a variety of DNA viruses in tissue culture in parallel with 9-(beta-D-arabinofuranosyl)adenine (ara-A). This evaluation revealed that xylo-G, xylo-GMP, and c-xylo-GMP were all moderately active but less effective than ara-A. When the four compounds were administered intracerebrally as a treatment for herpes virus, type 1 induced encephalitis in mice, c-xylo-GMP exhibited superior activity to that shown by the other three. When administered intraperitoneally, c-xylo-GMP was found to have a therapeutic index of about 4, which is less than that for ara-A (approximately 30) in the same system.