Purpose: To qualitatively and quantitatively compare the cytotoxic potentials of five different one-step self-etching adhesives: Prime&Bond One-Select (PB-OS), Optibond All-in-One (OB-AIO), G-Bond (GB), Clearfil Universal Bond (CUB), Single Bond Universal (SBU). Materials and Methods: During the first stage of the study, the cytotoxic activities of the test materials were evaluated qualitatively using the direct contact method. In this method, the test materials were placed directly into a monkey kidney epithelial cell culture medium. Reaction zones which occurred in the culture medium were evaluated, in addition to the density and changes in the morphology of the cells. During the second stage, the cytotoxic potential of four different dilutions (1%, 0.1%, 0.01%, 0.001%) of the test materials on L929 rat fibroblast cells was quantitatively evaluated at three different time periods (24 h, 48 h, 72 h) with the MTT tetrazolium-based assay. Results: In the first stage, a zone exceeding 1 cm was observed around or below SBU, CUB, GB and OB-AIO. In PB-OS, the zone borders were approximately 1 cm. In the second stage after the MTT assay, CUB was the most cytotoxic after 24 h, GB and SBU after 48 h, and OB-AIO after 72 h. Conclusion: All adhesives tested showed different degrees of cytotoxicity, which statistically significantly increased with dose. Changes were seen related to time. Keywords: cytotoxicity, direct contact method, MTT, one-step adhesives, self-etching