Since the earliest days of disinfectant testing two different methods have to be distinguished: one group simulating practical circumstances the other one allowing the determination of germ reduction, disregarding any practical conditions. At present the assessment of disinfectant substances is carried out according to the results of several methods, done in different countries. Usually standardised suspension-tests with no correspondance to-in-use conditions are done. These are for example the determination of the Rideal-Walker and the Chick Martin coefficient and the capacity test by Kelsey and Sykes as well. Results obtained by the use-dilution method (A.O.A.C.) are also just of theoretical value. The British Standard-Test 3286:1960 and the suspension-test of the Committee on Phytopharmacy (Netherlands), to be preferred in order to state germ reductions, do not imitate particulars of practice either. After establishing recommendations for the various disinfectant procedures, the unification of the test methods on international level should be the next aim. Besides evaluation of disinfectant substances (as it is done by suspension-tests) the testing of disinfectant procedures under conditions, copying those of real life ought to be required. The results of such in-use model tests should answer all the questions arising, for instance, what the concentrations or the exposure-time should be. We would plead in favour of testing disinfectant substances and procedures in two steps - preliminary testing and main tests under in-use conditions for each new preparation. The base for all these tests is the selection of a suitable inactivator.