In a previous study, 3-amino-4-[4-(dimethylamino)phenyl]-4,5-dihydro-1,2,5-thiadiazole 1,1-dioxide (DPTD), which is five-membered cyclosulfamide, was synthesized and structurally characterized. The aim of this study was to investigate the cytotoxic and genotoxic effects of DPTD on cultured human lymphocytes in the presence and absence of a metabolic activation system (S9 mix). The cytotoxicity and genotoxicity of DPTD in human peripheral blood lymphocytes were examined by using chromosomal aberration (CA) and micronucleus (MN) tests. Mitomycin-C (MMC) for cultures without S9 mix and cyclophosphamide monohydrate (CP) for cultures with S9 mix were used as positive controls. The cultures were treated with DPTD (45, 90, and 180 µg/mL) in the absence and presence of S9 mix. The cells were also co-treated with DPTD together with MMC or CP. DPTD showed cytotoxic activity due to decreases in mitotic index (MI) and nuclear division index (NDI) in the absence and presence of S9 mix. DPTD also increased the CAs, aberrant cells with CAs and MN values in cultures with and without S9 mix. When DPTD and MMC or CP were used together, lower MI and NDI values and higher CA and MN values were found than those DPTD treated alone. Both DPTD and its metabolites have cytotoxic, cytostatic and genotoxic potential on human peripheral blood lymphocyte cultures under the experimental conditions. Furthermore, co-treatment of DPTD and MMC or CP can cause more cytotoxicity and genotoxicity. Our results indicated that the use of DPTD with other chemotherapeutic drugs may display more effective results.