Four contract and two U.S. Environmental Protection Agency laboratories participated in round-robin tests using Daphnia magna as the test animals. The purpose was to determine if methods set forth in the 'Proposed Standard Practice for Conducting Renewal Life Cycle Tests with Daphnids' (ASTM Drafts 2 and 4 with modification) are adequate for routine laboratory testing. Silver and endosulfan were the toxicants used. The 48-hacute test used for determining EC50 values and estimating initial concentrations for the chronic test has been used for many years with good results, and this round robin confirmed its value.