Respiratory inhibitors exert a potentially biocidic action on all aerobic organisms. Therefore the systematic search for them and the elucidation of their mechanism of action is of great importance. The non-phosphorylating electron transfer particles (ETP) according to Crane et al. are a suitable test object for this aim. A standardized preparation of ETP from beef heart is described and characterized enzymatically. The ETP show a variety of advantages in comparison with other systems; their field of application is broad. Owing to the absence of bypasses of the respiration, particles from animals are more suitable than those from plants for the search for fungicides and herbicides which inhibit the respiratory chain. With relatively little expense, the two standard assay systems proposed (NADH oxidase and succinate-cytochrome c oxidoreductase) pick up with certainty all inhibitors of the main pathway of the respiratory chain; they are useful for screening studies. The system proposed is also suitable for the analysis of the exact site and the mechanism of action of respiratory inhibitors. Furthermore, ETP can be used to obtain parameters of the hydrophobicity of drugs as well as for toxicological studies.