The toxicity to Daphnia magna of a mixture of 14 aquatic pollutants with several chemical structures and probable modes of action was determined. The joint effect of this mixture on acute mortality was compared with the effects on inhibition of reproduction. The joint toxicity in the reproduction test was lower in comparison with the mortality test. This study supports the idea that the potential for addition is reduced when more specific sublethal toxicity criteria are studied. Although the joint toxicity at sublethal level is lower than at lethal level, the toxicity of the mixtures remains much higher than that of the individual chemicals and is still near concentration addition.