Seven arabinosyl, 2'-deoxyribosyl, and ribosyl pyrrolo[2,3-d]pyrimidines were evaluated in vitro for activity against human cytomegalovirus and for cytotoxicity in primary and established cell lines of human origin. The parent ribosyl analogs exhibited little antiviral selectivity owing to high cytotoxicity. In contrast, ara-tubercidin, ara-toyocamycin, ara-sangivamycin, and deoxysangivamycin exhibited selectivity between antiviral effect (measured by plaque or titer reduction or both) and cytotoxicity (measured microscopically and by incorporation of radioactive precursors into DNA, RNA, and protein). The selectivity (in vitro therapeutic indexes) for these four compounds ranged from 2 to 40. The two sangivamycin analogs were the most potent and selective. Ara-sangivamycin, for example, inhibited virus replication 10(5)-fold at a concentration (10 microM) which produced only partial inhibition of cell growth and labeled precursor incorporation. The four arabinosyl and deoxyribosyl nucleosides appeared to act by inhibition of viral DNA synthesis as quantitated by DNA-DNA dot blot hybridization. These four analogs also were tested for activity against two strains of type 1 herpes simplex virus by a plaque reduction assay. Unexpectedly, all compounds inhibited herpes simplex virus to a lesser extent than human cytomegalovirus.