While several studies have been reported on the physical testing of dentine bonding agents, very few studies have been involved with their biological evaluation. This report describes a new in vitro method for assessing the cytotoxicity of these materials through dentine simulating their clinical use. An experimental bonding agent, now marketed as Tripton by ICI Dental, Scotchbond 2 and GLUMA were exposed to BHK-fibroblasts through 100 and 500 microns slices. All materials were significantly cytotoxic compared with controls through both thicknesses of slice. The ICI Dental experimental material was the least toxic through 100 microns slices (68.8 per cent of controls) followed by Scotchbond 2 (54.2 per cent) with GLUMA the most cytotoxic (26.9 per cent). However, with 500 microns slices values for the ICI Dental experimental material and GLUMA were similar (70.8 and 66.6 per cent respectively), with Scotchbond 2 exhibiting greatest toxicity (52.6 per cent). The results indicate that these materials are considerably more cytotoxic than most restorative materials which have been tested using a similar system. It is anticipated that all materials, in particular Scotchbond 2, could cause severe reactions in deep cavities in vivo in the absence of an adequate lining.