A series of [[(guaninylalkyl)phosphinico]methyl]phosphonic acids, 2, was synthesized and tested as inhibitors of human erythrocyte purine nucleoside phosphorylase (PNPase). The target (phosphinicomethyl)phosphonic acids 2 were synthesized in six or seven steps from alkenylphosphonates 4. The latter were converted to the intermediate alkylmesylates 9 in a series of steps that included (1) conversion of the diethyl phosphonates 4 to the (phosphinoylmethyl)-phosphonates 7 and (2) conversion of the terminal double bond of [(alkenylphosphinoyl)methyl]-phosphonates 7 to the alkylmesylates 9. The pure 9-isomers 2 were obtained by alkylation of 2-amino-6-(2-methoxyethoxy)-9H-purine with alkylmesylates 9 followed by hydrolysis of the protecting groups with concentrated hydrochloric acid and ion exchange chromatography to give 2 as hydrated ammonium salts. The most potent inhibitor of human erythrocyte PNPase, [[[5-(2-amino-1,6-dihydro-6-oxo-9H-purin-9- yl)pentyl]phosphinico]methyl]phosphonic acid (2b), was a multisubstrate analogue inhibitor with a Ki' of 3.1 nM. Optimum PNPase inhibitory activity required the presence of zinc ions in the assay medium. These potent inhibitors of PNPase exhibited only weak activity against human leukemic T-cells in vitro.