The detection of illegal growth promoters in cattle urine by conventional immunoassays, such as radioimmunoassay (RIA) and enzyme‐linked immunosorbent assay (ELISA) has been extensively described. However, improved speed and simplicity, with the development of a fast and simple on‐site test as the ultimate goal, remains an interesting immunochem‐ical challenge. This paper describes two approaches to this problem. To avoid time‐consuming sample preparations, attempts were made to omit the extraction and hydrolysis step. Secondly, instead of the classical microtiter plate as the solid phase, magnetizable beads, which provided a solid phase that was dispersed throughout the sample, were used. In this design, the diffusion limitations of the free immunoreagents to the solid‐phase‐bound immunoreagent (which slow down the kinetics of the antibody‐antigen reaction) were circumvented. This principle was applied to the detection of the β‐agonist clenbuterol and the anabolic steroid nortestosterone in cattle urine. The assay of unextracted urine for the presence of nortestosterone failed due to matrix effects. For clenbuterol, a magnetic particle‐based immunoassay on unextracted urine samples was developed. The test, which can be performed within 90 min using pre‐coated beads, compared favourably to microtitre plate ELISA (with a4h incubation) and to a commercial RIA. The results on test samples were confirmed by gas chromatography/mass spectrometry.