Infections of pacemaker electrodes and/or aggregate pockets are usually caused by staphylococci, primarily Staphylococcus epidermidis. From in vitro experiments it can be demonstrated that staphylococci are able to adhere to the plastic electrode sheath, to multiply there, and to form microcolonies. Furthermore, the staphylococci produce a slimy amorphous substance with which they are completely covered after a 24-hour incubation. No difference was noticed between the in vitro experiment situation and that occurring in pacemaker infection in humans. This slimy material may be responsible for the maintenance of the infection and the protection of the enclosed bacterial cells against natural host defense mechanisms and antibiotic treatment.