AbstractThe effects of limiting nutrient to algal toxicity tests are discussed. It has been shown, through theoretical derivation, that EC50values will vary with changes of nutrient concentration even if the effects of limiting nutrient and limiting toxicant on algal growth are independent. The variation in EC50's is governed by the degree of correlation between two variables: the tolerance and the activation level. The proposed theory shows a potential application for making a comparison of results obtained from different laboratories or methods and for applying laboratory results to field conditions.