The precision of the 96-h LCs0 test with the eurohaline copepod Nitocra spinipes was evaluated from a data set containing 85 tests with the reference toxicant potassium bichromate conducted at 12 laboratories during a 12-year period. The overall mean was 30 mg/L as K2Cr207 with a coefficient of variation (CV) of 47% ( N = 85). Since salinity differed between tests (7-33 oioo), it explained part of the variation ica. 25%; Pearson two-tailed test r = 0.49; p = 0.0000; N = 85). The effect of salinity on Cr(V1) toxicity was confirmed in an experiment with 4 salinities (3.4, 6.8, 14.6, and 27 o/oo) and was explained to 99% by the equation 96-h LC50 (mg/L) = 2.58 + 1.79 x salinity (oleo).Intralaboratory CVs for corresponding salinities ranged from 8 to 35% (repeatability). Interlaboratory CVs a t two salinities were 42% ( N = 11) at 7-9 oioo and 47% ( N = 4) a t 15-16 doe. 0