Only some few systematic studies have appeared in the literature relative to the interference of drugs with clinico-chemical screening and rapid tests. This question is dealt with in the light of in vitro and in vivo investigations of the interference of selected drugs with screening test reagents produced in the G.D.R. In contrast to speculations in the secondary literature, a practice-relevant interference of the drugs under investigation with the test procedures was an exception. In this sense, our following findings are of importance: --inhibitory action of ascorbic acid and sodium fluoride on the demonstration of glucose by Biophan G, --false positive indication of Biophan PAS in case of high concentrations of sulphamerazine, --interference of ascorbic acid with the Biophan P test.