In this study we investigated the effects of two guanine derivatives, 9-benzyl- (I) and 7-benzyl-8-bromoguanines (II) on the proliferation of human T-cell leukemia and T-cell lymphoma, normal human peripheral blood mononuclear cells (PBMC), and mouse Th1 (pGL10) and Th2 (D10.G4.1) clones. We also assessed their effects on cytokine production (IL-3, IL-10 and IFN-gamma) in PBMC, T-cell lymphoma, HUT78 (IL-2), and murine Th1 (IL-2) and Th2 (IL-4 and IL-5) clones. These compounds were synthesize as analog of known inhibitors of purine nucleoside phosphorylase (PNP) 8-amino-9-benzylguanine. These compounds suppressed proliferation of human leukemia MOLT-4 cells, human cutaneous lymphoma HUT78 cells and normal PMBC. Compound II was a significantly more potent inhibitor than compound I. Exogenous recombinant human IL-2 reversed the anti-proliferative effects of both compounds on HUT78 cells. These compounds had low toxicity to human EBV-transformed B-lymphocytes. Both compounds suppressed the production of IL-2 by activated human HUT78 cells, IFN-gamma by PBMC and did not affect IL-3 and IL-10 production in PBMC. Compound I inhibited anti-CD3-activated IL-2 secretion from the murine Th1 clone. The murine Th2 clone was less sensitive to both compounds as compared with Thl. The production of IL-4 and IL-5 by this clone was not suppressed. Thus, it has been shown that not only 9-substituted guanines but also their 7-isomers selectively inhibit T-cell functions and both selectively inhibit Th1-related cytokines secretion.