Numerous factors affect the acute toxicity of drilling fluids (muds) to mysids (Mysidopsis bahia). Source, composition, and age of drilling fluid sample; preparation of test material; condition of test animals; and skill and experience of the people conducting the tests can influence test results. Despite these confounding factors, our intralaboratory variation of median lethal concentrations (96-h LC50s) for six tests with a laboratory-prepared generic drilling fluid was within a factor of two; interlaboratory variation for seven commercial laboratories that tested the same generic drilling fluid was within a factor of four, the same as reported in the literature for acute toxicity tests with single chemicals. The presence of petroleum hydrocarbons in drilling fluids greatly increases toxicity and, because toxic, volatile fractions may be lost, variability of results from tests with petroleum hydrocarbon-contaminated drilling fluids may be greater than that stated above.