The cytotoxicity of a newly developed dentin bonding system was examined in African green monkey kidney cells and human embryonic lung cells. A tissue culture agar overlay procedure, a longitudinal study, and a cell replication assay were employed to evaluate cytotoxicity. Several of the components were discovered to be cytotoxic when tested individually in vitro. However, when used in the prescribed manner, little or no cytotoxic effect was elicited. The results of this study support further testing of these materials in vivo.