A comparative simultaneous evaluation of the Optimate and the TDX analyzers was performed according to guidelines proposed by the National Committee for Clinical Laboratory Standards. Statistically significant (P less than 0.05) better precision was demonstrated for the TDX analyzer with both commercial controls and patient pools. No statistically significant difference was found between the analyzers in terms of linearity, accuracy, and curve stability. Technologists judged the TDX to be superior in ease of start-up and shutdown procedures, loading and unloading, test changeover, time for a stat specimen, and time for an average run. In general, the TDX analyzer required less operator interaction for routine performance, but its consumable expenses were about 50% greater than those for the Optimate analyzer.