Experiments were done about the behaviour of 5 different strains of St. aureus (= St), E. coli (= Ec), Se. marcescens (= Se) and P. aeruginosa (= Pa) in aqua bidest. at 30 degress C and 40 degrees C. In the first part the smallest quantities of Standard-I-broth (Merck) were evaluated where the diverse bacteria-species are able to grow; it was found at 30 degrees C (in parenthesis at 40 degrees C): St. aureus 1750 (3750) microgram/dl, E. coli 1000 (1750) microgram/dl, Se. marcescens 2.5 (125) microgram/dl and P. aeruginosa 1250 (2000) microgram/dl. In the second part mixed cultures were formed in the following groups: St/Ec, St/Se, St/Pa, Ec/Se, Ec/Pa, Se/Pa. In each case such quantities of broth were given into aqua bidest that the more fastidious bacteria was just able to grow. The initial colony count in both bacteria-species was log 3-4/ml. It was reduced to log 2-3/ml in one of these two species in a second part and in the other species in a third part of this experiment. Mixed cultures of St. aureus with gram negative bacteria resulted in reduced numbers of St. aureus; in case of reduced initial colony counts of E. coli, Se. marcescens or P. aeruginosa colony numbers of St. aureus were a little higher. In mixed cultures Ec/Se and Se/Pa at 30 degrees C colony numbers of E. coli and P. aeruginosa were reduced initial colony counts of Se. marcescens they were also a little higher. In a further part of this experiment such quantities of broth were given that the more fastidious bacterium was not able to grow. Here it died quickly; there were no differences to control groups with only one species.