We investigated the effect of tannic acid, a potent inhibitor of poly(ADP-ribose) glycohydrolase, on human viral gene transcription, by using chloramphenicol acetyl transferase (CAT) assay experiments transfecting Jurkat cells with CAT reporter constructs that contain the promoter region of human immunodeficiency virus (HIV) or of human T-cell leukemia virus type I (HTLV-1). The activity of HIV promoter induced by treatment with 12-O-tetradecanoylphorbol-13-acetate was suppressed by the addition of tannic acid. On the other hand, HTLV-1 promoter activity induced by the p40(tax) expression plasmid was not affected by tannic acid treatment. Deletion analysis of the HIV promoter revealed that a 30-bp element located immediately upstream of NF-kappa B motifs was responsible for the suppressive effect of tannic acid. This was supported by the observations that the negative effect of tannic acid was introduced to tannic acid-non-responsive thymidine kinase promoter by the insertion of this element 5'-upstream of the promoter.