Two methods for measuring the effect of organic toxicants on the marine macroalgaEnteromorpha intestinaliswere compared. EC50data for the effect of C1−C8n−alcohols on neutral red dye retention and ion leakage were used to construct QSARs with log Kowas the only physicochemical parameter. The two QSARs were statistically indistinguishable suggesting that both procedures reflect the same non-specific narcotic effect on membrane integrity. The effect on ion leakage of compounds with more specific modes of action was also investigated. The herbicides diuron and diquat had no effect at concentrations which inhibit photosynthesis, the uncoupler 2,4-dichlorophenol was no more toxic than a narcotic of similar polarity, but the known high toxicity of the antifouling compound tributyltin was clearly reflected by the ion leakage response.