The bactericidal activity of three disinfectant standards has been determined by four national methods (the qualitative suspension test of the German Society for Hygiene and Microbiology DGHM, the quantitative suspension test of the Dutch Committee on Phytopharmacy, the use-dilution method of the A.O.A.C. and the Kelsey and Sykes' test) in order to compare these four testing techniques. In previous publications the results of these experiments and the degree of standardisation of the four methods have been tested. The purpose of the present study is to evaluate to what extent the results obtained by the four testing techniques do correspond. The bactericidal activity can be expressed as a germicidal effect. In theory (Tab. 1) the values of germicidal effect can be similar for the four testing techniques. In practice (Tab. 2 and 3) it has been found that with the suspension technique of the DGHM systematically higher values are obtained than in the other tests. These can not be compared to each other, because the medication times differ. Although the values of the germicidal effect obtained are widely different, it can happen that the minimum effective concentration determined by the four methods be identical as each of these methods applies different criteria. The authors have therefore calculated the minimum effective concentration which is required for the disinfectant standard by the four disinfectant testing techniques (Tab. 4). Even here, no correspondence has been found between the four methods. The lethal concentration of the disinfectant standards determined by the use-dilution method of the A.O.A.C. is thus 2 to 16 times those required by the Dutch suspension test. The difference in effective concentrations, however, is not constant and is not related to the type of disinfectant or the test organism. In consequence, there is no way of establishing a comparison between the results of the four methods with a view to setting up a conversion table. Each disinfectant testing technique has its own characteristics.