AbstractA protocol for determination of “no‐effect” concentrations on hatch and survival in the zebrafish (Brachydanio rerio) has been evaluated at five laboratories for variability using chromium (K2Cr2O7) and zinc (ZnSO4× 7H2O) as toxicants. Repeatability (r) and reproducibility (R), expressed as C.V. (coefficient of variation), for logarithmic median survival time ranged between 1% and 9% forrand between 1% and 29% forRdepending upon concentration of toxicant. Variability increased at high concentrations, especially for zinc. For logarithmic median hatching time, C.V. forrranged from 6% to 24% and forRbetween 11% and 27% for the various concentrations of chromium and zinc. Estimated logarithmic “no‐effect” concentrations on survival of zinc and chromium had a C.V. forrbelow 10% and a C.V. forRbelow 15% for zinc and below 25% for chromium. Zinc delayed the time to hatch at lower concentrations than those that reduced survival time, for the estimated logarithmic “no‐effect” concentrations of zinc on hatchrwas around 75% andRaround 100%. The “no‐effect” concentrations based on all nine or ten tests from the five laboratories were 15 mg Cr/L and 0.5 mg Zn/L. Because of the cumulative toxicity of chromium in fish the chronic “safe” concentration may be considerably lower, but the “no‐effect” concentration of 0.5 mg Zn/L for effects on hatching time is consistent with the chronic “safe” concentration found in other species of fish at the same