Environmental examination were carried out in 50 hospitals using "rodac-plates" (size: 21 cm2) for 7971 samples, to acquire a semiquantitative assessment of microbial contamination of surfaces. Up to 20 percent of the impression cultures showed uncountable numbers of colonies (more than 300/21 cm2). Therefore statements on the microbial content were not to be based on mean values alone. The actual state of surface contamination was assessed by the mean value of all plates with colony counts below 300 and by the median value. Additionally numbers of samples with colony counts below 10 and more than 300 were recorded. The mean value of all samples with less than 300 colonies/21 cm2 was 58. 21% had less than 10, 23% more than 300 colonies. In operating suites, delivery rooms, and premature wards the mean value was 30 colonies/21 cm2. Surfaces with less than 10 colonies amounted to 30 percent, with more than 300 to 5 percent. Samples from intensive-care units, from new-born-, and childrens wards had mean values of 50 colonies/21 cm2. Percentages of plates below 10 and above 300 colonies differed too much to be grouped. Surgical, internal, and gynaecological wards showed mean values of more than 60 colonies/21 cm2. 15 percent of plates had less than 10 colonies, 10 percent more than 300. Differentiations as to specifications of rooms and surfaces in regard to bacterial contamination have been made.