Transthoracic needle aspiration of the lung in guinea pigs with experimental pseudomonas pneumonia was evaluated. The number of bacteria in aspirates correlated well with that of bacteria in the lungs which showed diffuse pneumonia (10(7) inoculum group). The number of deaths of experimental animals increased together with an increase of the times of aspiration. This procedure was also investigated in 16 patients of pneumonia and 17 patients of pulmonary abscess. The isolation rate of pathogen from pneumonia was 31.3% and that from pulmonary abscess 58.8%. A higher isolation rate was obtained with purulent aspirates. Predominantly anaerobic bacteria were isolated, and in pulmonary abscess usually in association with other bacteria. This method could be applied with success, for determining responsible pathogens. Even the normal oropharyngeal flora such as alpha-Streptococcus could be identified as pathogens. The complication rate was relatively low (6 out of 33 patients, 18.2%) including hemoptysis as a major one and pneumothorax or bloody sputum as minor ones. Transthoracic needle aspiration was reevaluated in experimental and clinical materials and was found to be an excellent and safe method for determining the responsible pathogen of respiratory infection.