"INTRODUCTION Since 1945 [1] many scientists have advocated use of toxicity data in predicting potential damage to a receiving stream before damage occurred. Toxicity data have been used for predictive@ comparative@ and regulative purposes@ and the acute toxicity test has become a ""workhorse"" in monitoring pollution effects for management decisions even though the reliability of such tests has been questioned [e.g.@ 2@3@4@5@6@7]. In January 1981@ the U.S. Environmental Protection Agency (USEPA) distributed a draft document on the use of effluent toxicity testing in the second round of the National Pollutant Discharge Elimination System (NPDES) permit Issuance [8]. The USEPA proposed that a 24-hr screening test be conducted on a sample of 100% effluent. The proposed criterion for passing the screening test was 80% or greater survival of test organisms after a 24-hr exposure. If survival fell below 80%@ a 48-hr definitive test was to be conducted. Many Industries do not have toxicity testing capabilities@ and they usually contract laboratories to conduct tests for them when they are required to do testing. Many different protocols are available for conducting acute toxicity tests [e.g.@ 9@10@11@12@13]@ Although attempts have been made to standardize test methods@ differences still exist between methods@ even though some are minor. In addition@ possible differences exist in the interpretation and understanding of a test method by the people conducting a test. As a result@ toxicity test results obtained for effluents possibly may not be comparable from test to test conducted within a laboratory and among laboratories. The purpose of this research was twofold: (a) to compare the inter- and interlaboratory variation in toxicity test results using reference toxic ants and refinery effluent samples@ and (b) to evaluate the utility of the USEPA proposed toxicity tests for industry. Duplicate effluent samples from five refineries and a synthetic mixture were sent to three laboratories for toxicity testing over a period of six weeks."