Five bulk fill composite resins, including SDR, Tetric EvoCeram Bulk Fill (TEC), X-trafil (XTF), Sonic Fill (SF), Filtek Bulk Fill (FBF), were used in this study. Human dental pulp stem cells were cultured in 12-well culture dishes (3 x 104 cells per cm(2)) and stored in an incubator at 37 degrees C and 5% CO2 for 1 day. On days 1, 7, 14, and 21 of co-culture, viable cells were measured using a WST-1 assay. Lower cell viability was observed with XTF and SDR bulk fill composite resins compared to the control group during the WST-1 assay. Although bulk fill composite resins provide advantages in practical applications, they are limited by their cytotoxic properties.