Objective: This study was undertaken to characterize residual mercury content of dental wastewater lines and to determine the leaching potential of mercury and silver utilizing Toxicity Characteristic Leaching Procedure (TCLP) analysis. TCLP extraction determines the mobility of analytes in an acetic acid buffer solution. The concentration of regulated analytes in the extract determines the toxicity characteristic of a sample and therefore whether it is subject to disposal regulations under the Resource Conservation and Recovery Act (RCRA). RCRA limits for mercury and silver are 0.2 mg/liter and 5.0 mg/liter respectively. Methods: Segments of dental wastewater lines were obtained from three dental treatment facilities ranging in size from 30 to more than 100 dental chairs. Determination of residual mercury content was done with United States Environmental Protection Agency (USEPA) method 7471, TCLP extraction was done with USEPA Method 1311, mercury analysis of TCLP extracts was done with USEPA method 7470A (Cold Vapor Atomic Absorption Spectrometry), the analysis of Ag concentrations in the TCLP extract was done with USEPA method 6010 (Inductively Coupled Plasma-Atomic Emission Spectrometry, ICP-AES) and the digestion of extracts for ICP-AES analysis was done with microwave assisted acid digestion (USEPA Method 3015). Results: Residual mercury content averaged 29.6 grams/kg (range=0.710 grams/kg to 177 grams/kg, sd=55.4). TCLP analysis showed Hg levels in the leachate to average 0.1153 mg/liter (range=0.0179 to 0.396 mg/liter, sd=0.1069). Ag levels ranged from non-detectable levels (<0.05 mg/liter) to 0.537 mg/liter. Conclusions: Results show considerable amounts of residual mercury in wastewater lines from large dental clinics. TCLP analysis shows waste lines to be capable of leaching mercury over RCRA limits making them subject to hazardous waste disposal regulations under RCRA. Mercury in waste line sludge can be mobilized by the action of oxidizing line cleanser. This study was supported by a grant from the USEPA, IAG# number: DW17947929-01-0.