Previous studies have already demonstrated the protective role of vitamin C (ascorbic acid) in certain types of cancer. This study reports on the effects of vitamin C on arylamine N-acetyltransferase (NAT) activity and DNA adduct formation in a human bladder tumor cell (T24) line. The activity of NAT was measured using high-performance liquid chromatography (HPLC), by assaying for the amounts of acetylated 2-aminofluorene (AF) and p-aminobenzoic acid (PABA) and the remaining amounts of AF and PABA. T24 cells were used for examining NAT activity and carcinogen DNA adduct formation. The results demonstrated that NAT activity and 2-aminofluorene DNA adduct formation in T24 cells were inhibited and decreased by vitamin C in a dose-dependent manner. The apparent kinetic parameters (apparent values of Km and Vmax) from T24 cells were also determined with and without vitamin C cotreatment. The data also indicated that vitamin C decreased the apparent values of Km and Vmax from T24 cells.