In this final regulation, EPA ratifies approval of several test procedures for measuring the toxicity of effluents and receiving waters. The test procedures are commonly referred to as whole effluent toxicity or WET test methods. EPA also withdraws two WET test methods from the list of nationally-approved biological test procedures for the analysis of pollutants. This action also revises some of the WET test methods to improve performance and increase confidence in the reliability of the results. Today's action will satisfy settlement agreement obligations designed to resolve litigation over an earlier rulemaking that originally approved WET test methods.