Applications of electron data processing in all fields of medicine and especially in medical microbiology are increasing rapidly. It seems important to discuss a guide for using such complicated electronical systems, because, on the one hand, computer applications in medicine are desirable for scientific reasons, but on the other hand few very costly and sophisticated projects had to be cancelled, after a shorter or longer time of implementation by reason of lack of co-operation of medical collaborators. This paper puts on discussion such a guide for using electronic data processing in medical microbiology. It is proposed to put through the following tasks before introducing a computer system into a microbiological institution: a system analysis of the status quo, an objective determination of the set state, a system design and a cost-benefit analysis. Those tasks are described in detail and are illustrated to some extent by graphics and tables from own research for preparing a data processing system for medical microbiology. It is emphasized that beside hard- and software criteria of the desired computer system one has to pay attention to the problems of man-machine engineering. The paper finishes with concrete propositions of proceeding when the computer system is implemented and shows possibilities of scientific data evaluation of a microbiological data base.