This International Standard specifies a method for the determination of the growth inhibition of unicellular green algae by substances and mixtures contained in water or by waste water. This method is applicable for substances that are easily soluble in water. With modifications to this method, as specified in ISO 14442 and ISO 5667-16, the inhibitory effects of poorly soluble organic and inorganic materials, volatile compounds, heavy metals and waste water can be tested. Monospecies algal strains are cultured for several generations in a defined medium containing a range of concentrations of the test sample. The test batches are prepared by mixing appropriate quantities of growth medium, test sample, and an inoculum of exponentially growing algal cells. The test batches are incubated for a period of (72 ± 2) h during which the cell density in each test solution is measured at least every 24 h. Inhibition is measured as a reduction in specific growth rate relative to control cultures grown under identical conditions. Annex A contains a screening test using various types of test containers and waters of equivalent purity. Annex B contains an alternative test procedure using algae from algal beads. The following modifications have been made with respect to DIN EN ISO 8692:2005-01: a) a definition of the term "lowest ineffective dilution" (LID) has been included; b) Annex B specifying an alternative test procedure with algae from algal beads, with direct measurement of algal growth in spectrophotometric cells has been included; c) a new Annex C covering the procedure for immobilization of algae in alginate beads has been included (see 5.1); d) the standard has been editorially revised. The underlying International Standard has been prepared by ISO/TC 147/SC 5/WG 5 "Toxicity - Algae and aquatic plants", in collaboration with Technical Committee CEN/TC 230 "Water analysis", the secretariat of which is held by DIN. The responsible German committee is NA 119-01-03-05-01 AK "Biotests" ("Biotests") at NA 119-01-03 AA "Wasseruntersuchung" ("Water Analysis") at the NAW.