An interlaboratory comparison of toxicity tests with the polychaete Neanthes arenaceodentata was conducted to determine the amount of variability expected with this test. Six laboratories each conducted two 28-day flow-through tests, one with silver nitrate (AgNO3) as the toxicant and the other with the pesticide endosulfan. Muffled sand (from unpolluted beaches) was used as sediment in the tests. The mean 28-day median lethal concentrations (LC50s) (based on measured concentrations) for the participating laboratories were 165 plus or minus 52micrograms/L and 106 plus or minus 24micrograms/L for the tests with silver and endosulfan, respectively. The ratio of the highest to the lowest LC50 value was 2.33 for silver and 1.81 for endosulfan. Therefore, the variability between laboratories was low for this test.