Abstract: The invention concerns a method for determining the toxicity of water-soluble substances or of substances which can be mixed with water and thus obtaining quantitative information concerning the toxic effect of said substances. The invention helps to reduce the number of tests, necessary hitherto, performed on mammals. According to the method, at least two measurements determine the variation in mobility and/or size of test organisms in an aqueous medium, under the influence of at least one toxic substance. A device for carrying out the method comprises an image-detection unit which detects the object and sequentially records images of this object in at least one plane at predetermined time intervals. Arranged downstream of the image-detection unit is an evaluation arrangement with a computer and display devices which statistically evaluate the mobility and/or size of the organisms in relation to the existing toxicity.