Mutagenicity of single compounds of dental resinous materials has been investigated on many occasions before, but the induction of mutagenic effects by extracts of clinically used composites is still unknown. Here, cytotoxic effects and the formation of micronuclei were determined in V79 fibroblasts after exposure to extracts of modern composite filling materials (Solitaire, Solitaire 2, Tetric Ceram, Dyract AP, Definite). For cytotoxicity testing, test specimens were aged for various time periods (0, 24, and 168 h), and V79 cells were then exposed to dilutions of the original extracts for 24, 48, and 72 h. The ranking of the cytotoxic effects of the composites according to EC50 values after a 24-h exposure period was as follows: Solitaire (most toxic)=Solitaire 2<Tetric Ceram<Dyract AP<Definite (least toxic). Cytotoxicity was independent of the period of aging for each composite, but varied with exposure periods. The cytotoxic effect of Solitaire increased about two-fold between exposure periods of 24, 48, and 72 h, no changes were observed with Solitaire 2, and cytotoxicity of Tetric Ceram, Dyract AP, and Definite was reduced. Even eight-fold diluted original extracts of freshly mixed Solitaire test specimens increased the numbers of micronuclei about 10-fold, and Solitaire 2 was slightly less effective. The mutagenic effects of these materials were reduced in the presence of a rat liver homogenate (S9). Weak increases of the numbers of micronuclei were detected only with undiluted extracts of Tetric Ceram and Dyract AP, but Definite was not effective. Our findings suggest that mutagenic components of biologically active composite resins should be replaced by more biocompatible substances to avoid risk factors for the health of patients and dental personnel.