Acute toxicity tests were conducted to assess the sensitivity of laboratory cultured Tubifex tubifex (Annelida, Clitellata), as an intralaboratory quality assurance requirement for the test organisms used in sediment chronic bioassays. Worms were exposed to cadmium, copper, and chromium in water-only 96-h tests, in single metal exposure, over a three-year period. The lethal concentration for the x% of the exposed population (LC(x)), the No-Observable-Effect Concentration (NOEC) and the Lowest-Observable-Effect Concentration (LOEC) were estimated. Based on LC(50) values, the rank of toxicity was Cu(+2)>Cd(+2)>Cr(+6). Sensitivity remained relatively constant during the research period and was comparable with the sensitivity of the same culture stock examined more than 10 years ago. This observed consistency in sensitivity indicates that comparisons between different sediment chronic bioassays conducted during the study period, using T. tubifex worms from the same culture stock, was appropriate. It is necessary to control both worm biomass and age for the acute test in order to standardize the test-organism conditions in intralaboratory quality assurance programs.