Organic concentrates of the River Rhine water were tested for toxicity with a 15 minute test (MicrotoxTM) on luminescent bacteria (Photobacterium phosphoreum, Bacteria). A total number of 18 stations in the Dutch lower Rhine area were sampled five times with an interval of one month (April 1988–August 1988), whereas two stations located the most apart (Lobith and Maassluis) were sampled with two-week intervals in the period between January and October. The river samples were concentrated by neutral absorption on XAD resins, followed by elution with acetone. Toxicity is expressed on a relative scale. The relative toxicity is in the range of approximately 3 to 18, which means that the organic toxicants present in Rhine water had to be concentrated between 55-and 330-fold. During transport in The Netherlands, the dissolved organic toxicity contained in the water of the River Rhine decreases gradually, either as a consequence of degradation, or by adsorption to newly formed particulates. In the industrialized areas of the lower Rhine in The Netherlands a slight, but temporary increase of toxicity is observed. Since water quality improved strongly with respect to inorganic (heavy metal) pollution, the organic toxicity reflects the actual pollutional status more closely than in former days. The speed and simplicity of the combination of XAD concentration and Microtox toxicity determination enables large-scale monitoring studies on organic toxicants in moderately polluted surface water.