The first series of 5'-sulfamoylated carbocyclic purinyl nucleosides was synthesized and tested for antitumor and antibacterial activities. The target compounds were formed by reacting the 2',3'-acetonide-protected carbocyclic nucleosides with sulfamoyl chloride, followed by deprotection. The agents were tested for cytotoxic activity against P388 mouse leukemia cells. Two compounds, 5'-sulfamoyl carbocyclic adenosine (2) and 5-sulfamoyl-8-aza carbocyclic adenosine (6) exhibited IC50 values as low as 62 and 15 nM, respectively. These analogs inhibited protein biosynthesis and slowed down DNA and RNA biosyntheses in the P388 cells. None of the target molecules were as potent against Escherichia coli as they were against the tumor cells. Also, in cell-free systems, agents 2 and 6 were more effective inhibitors of protein synthesis in rabbit reticulocyte lysate than in E. coli. These new carbocyclic derivatives appear to be somewhat selective for eukaryotic over prokaryotic cells in affecting translation.