In the analysis of combined effects two reference concepts are currently considered as equally valid for the assessment of mixture toxicities: these are LOEWE additivity (concentration addition) and BLISS independence (response addition) (Greco et al., 1995). The aim of this study of 137 binary mixtures of pesticides and surfactants using an algal biotest was to find rational procedures for the assessment of mixture toxicities in the aquatic environment. By introducing an index on prediction quality the quantitative relationships between predicted and observed effects are evaluated for each concept. It is shown that LOEWE additivity leads to good predictions of mixture toxicities for most combinations, whereas BLISS independence tends to underestimate mixture toxicities. By this it is reaffirmed that there is a solid basis for forthcoming regulatory activities on mixtures of chemicals.