3-Deazaneplanocin, a new carbocyclic analog of adenosine, was synthesized as an inhibitor of S-adenosylhomocysteine hydrolase. The Ki of 3-deazaneplanocin for a purified hamster liver preparation of S-adenosylhomocysteine hydrolase was 5 X 10(-11) M, making this inhibitor 250-fold more potent than the previously known most potent inhibitor of this enzyme, 3-deazaaristeromycin. Inhibition was competitive with the substrate adenosine. Human promyelocytic leukemia (HL-60) cells treated with 10(-5) M 3-deazaneplanocin showed a pronounced elevation in S-adenosylhomocysteine which was 4-fold greater than that produced by an equimolar concentration of 3-deazaaristeromycim. This effect preceded a moderate reduction in cell growth and viability following continuous exposure for 6 days. Cellular differentiation as monitored by the reduction of nitroblue tetrazolium was not markedly affected except after 4 days exposure to 10(-5) M 3-deazaneplanocin where 60% of the viable cells were positive. These results indicate that 3-deazaneplanocin may have therapeutic potential as an anticancer or antiviral drug.