A series of 8-amino-9-substituted guanines was synthesized and their activity evaluated against human purine nucleoside phosphorylase (PNP). All compounds were found to be potent inhibitors of human PNP (IC50s: 0.17-126 μM). They were also selectively cytotoxic to MOLT-4 lymphoblasts in the presence of a nontoxic amount (10 μM) of the PNP substrate, 2′-deoxyguanosine (GdR). The most potent of these analogs, 2,8-diamino-1,9-dihydro-9-(2-thienylmethyl)-6 H-purin-6-one (8-amino-9-(2-thienyl-methyl)guanine; PD 119,229) has an IC50 of 0.17 μM (Ki=0.067 μM), significantly more potent than the known standard, 8-aminoguanosine (IC50=1.40 μM). Thus it represents the most potent PNP inhibitor known to date when tested without limiting the concentration of inorganic phosphate. © 1987 Birkhäuser Verlag.