A novel testing procedure has been developed with the aim to replace the traditional LD50 test in vertebrates by a method using a non-pain sensitive organism. Several years of practical experience have proven this method to be a rather quick, simple, inexpensive, outstandingly well reproducible and reliable experimental technique which yields an estimate for the acute toxicity of drugs, environmental chemicals, solvents, food additives, pesticides, industrial and waste products, and the like. The model is equivalent to the customary LD50 test in mice, rats and other laboratory animals. The yeast test, as it has been briefly named, employs ordinary yeast (Saccharomyces cerevisiae) in a thermostatized incubation mixture with nutrients and trace elements. The test substance is added to this mixture by increasing concentration, and the effect upon the growth rate of the yeast cells is monitored at 30, 90, 150 and 210 min after beginning the experiment by counting the cell number, either in a simple counting chamber under the microscope or, more conveniently, by using an electronic Coulter counter. The effect is expressed as percent growth of the cells in relation to the untreated control. Evaluation of the experimental data leads to a general toxicity parameter, the mean inhibitory concentration or IC50 value of the compound under test. Hitherto it was found that the IC50 values of approx. 160 common drugs and other chemicals correlate well with the known LD50 values found in animals with the same substances.