PURPOSE: Current in vitro cytotoxicity tests for dental casting alloys may be inadequate because they use a short-term test (generally 168 h) for a material that is present in the mouth for years. A modified in vitro test was used to test the cytotoxicity of 8 dental casting alloys and 3 pure metals. This modified test attempted to simulate the long-term effects of dental casting alloys in the mouth. MATERIALS AND METHODS: Alloys were tested in cell culture either immediately after polishing (initial) or after 10 months of conditioning in a biologic medium containing serum proteins. The cytotoxicities of the 2 conditions were then compared. Alloys were repolished to assess the reversibility of the 10-month conditioning. RESULTS: Alloys with little initial cytotoxicity showed no 10-month cytotoxicity. Alloys with significant initial toxicity showed significantly less toxicity at 10 months. No alloy increased in toxicity after conditioning. Repolishing multiple times did not necessarily reestablish the initial cytotoxicity response. CONCLUSION: The results indicate that short-term cytotoxicity tests may not accurately measure the long-term cytotoxicity of these types of alloys.