In spite of the successful treatment of bacteriological infections combined with progress in hospital hygiene there remain still problems of infective hospitalism in certain sections of the hospital. Generally, the causative bacteria are nosoparasites not being dangerous to human beings in good condition. From this point of view Pseudomonas aeruginosa and Serratia marcescens are the most important species. A successful challenge against infective hospitalism is only possible under the supervision of the bacteriologist. It is his task to isolate and differentiate the bacteria and to carry out the sensitivity tests against antibiotics. Besides this, he has to treat epidemiological and hygienic problems. Special kinds of methods sometimes have to be used to find out the sources and the routes of spreading of the infections in a hospital e.g. bacteriophage typing, bacteriocin typing and analysis of the antigenic structure. Likewise, the transmission of bacteria by air has to be studied and continuous controlling measures and monitoring are required. The bacteriologist is also responsible for preparing hygiene instructions. There always has to exist a good cooperation between the bacteriologist, the clinical doctors, the nurses and the technical staff. The whole problem can only be handled by special teams.