This document specifies a method for the determination of the inhibition of growth of the unicellular marine algae Skeletonema sp. and Phaeodactylum tricornutum by substances and mixtures contained in sea water or by environmental water samples (effluents, elutriates, etc.). The method can be used for testing substances that are readily soluble in water and are not significantly degraded or eliminated in any other way from the test medium. NOTE With modifications, as described in ISO 14442 and ISO 5667-16, the inhibitory effects of poorly soluble organic and inorganic materials, volatile compounds, metal compounds, effluents, marine water samples and elutriates of sediments can be tested.