The cytotoxicity of four self-etching primer/adhesive systems (Clearfil(R) SE Bond, Clearfil(R) Protect Bond, Mac Bond(R) II and FL(R) Bond) was tested against L929 fibroblasts. The primer or adhesive component of each adhesive system was diluted serially with the culture medium at a ratio of 1: 1,000 and 1: 4,000 (v/v). Cytotoxicity was identified by adding L929 cells in 24-well culture plates at an initial density of 35,000 cells mL(-1). The cells were maintained for 5 days; every 24 h, the medium was changed with fresh medium containing specific dilutions of the primer or adhesive components of the test materials. Cytotoxicity was assessed quantitatively at 24, 48, 72, 96 and 120 h. Physiological and pathological cellular changes as well as reactions and growth of the cell cultures were examined under an inverted microscope. All self-etching systems were found to be cytotoxic to varying degrees; more pronounced toxic effects were observed at lower dilution (1: 1,000 [v/v]). The adhesive components of Mac Bond(R) II and FL(R) Bond showed the highest cytotoxicity at 1: 1,000 (v/v). The primer and adhesive of Clearfil(R) SE Bond, the primer of Mac Bond(R) II and the antibacterial monomer (MDPB)-containing Clearfil(R) Protect Bond (at 1: 4,000 [v/v]) were relatively less cytotoxic.