In clinical immunology the overall assessment of cellular immunocompetence has become a subject of increasing interest. Beside the development of various sophisticated in vitro methods, simple skin tests remain a basic instrument. Contact sensitization to dinitrochlorobenzene (DNCB) has proved to be a reliable parameter in detecting impaired cel-mediated immunity, in that previous exposure to this antigen is rare and the epidermal application induces a primary immune response in the overwhelming majority of normal individuals. The morphology of the skin reactions corresponds to that of contact eczema (allergic contact dermatitis). A review of the literature shows that various techniques of the test are applied which complicate the interpretation and comparison of different results. The aim of this study was to report our own experiences with DNCB contact sensitization in normal individuals and in patients with Crohn's disease and complex glomerulonephritis and to discuss practical problems in test application; in particular the differentiation between toxic and allergic (delayed hypersensitivity) dermatitis, the choice of the sensitizing and rechallenge dose, the principles of recording successful sensitization, the influence of the inflammatory response upon sensitization, side effects, and precautions. Finally a modified test system is recommended.