An attempt was made to define the value of the bacteriological examination of tracheal aspirations in patients on artificial ventilation. Quantitative cultures of lung biopsies, taken immediately after death, were made on 21 patients in an intensive care unit. The results were compared with the clinical signs, antibiotic therapy and quantitative cultures of tracheal secretions taken before and very soon after death. A comparison of the results shows that microbial species present in tracheal secretions at a level equal to or greater than 10(7)/ml were found in 53 percent of lung biopsies. However, the predominant bacteria found in the lung biopsies were not necessarily similar to those in the tracheal secretions. Also, bacteria minimally present or completely absent in tracheal secretions could be found in lung biopsies. Therapeutic decisions can only be made on clinical and radiological signs; bacteriological examination may be a help in the choice of drugs once such treatment has been decided upon.