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Endodontic root-filling material is brought into direct contact with apical tissues when finishing endodontic therapy. Endodontic two-component materials develop cytotoxic effects during the phase, as shown in different in-vitro studies. Insufficient specificity of these tests could be eliminated by using bone cell cultures. The toxicity of seven endodontic root-filling materials towards osteoclasts in primary cell culture were investigated. Osteoclasts initially reacted to freshly mixed endodontic materials by losing their physiological properties (surface adherence), longer exposition on the toxic agents led to cell lysis. To quantify these light-microscopical phenomena, the content of lactate dehydrogenase (LDH) in the adherent cells as well as in the medium of the cultures were determined photometrically. All the examined two-component materials induced a distinct loss of intracellular LDH activity with a proportional uprise in the media. Calibration showed a positive correlation of LDH content and cell number. The results could be interpreted as the survival rate of osteoclasts after incubation with endodontic filling materials. All tested materials--except for gutta-percha--showed a distinct toxicity towards osteoclasts in primary cell culture during the first 24 hours. Osteoclasts proved to be sensitive indicators for cell-adverse toxicity during the in-vitro tests of endodontic root-filling materials.