SummaryIn more than half the cases studied, the authors have observed a discrepancy between the bacteriological examination of the sputum and that of the secretion obtained aseptically from the bronchi.They criticize the practice of performing routine susceptibility tests to antibiotics on the sputum.They attract attention on the high rate of incidence of sterile bronchial smears, which witnesses the resistance of bronchial mucosa towards infection. They analyse the microbiological flora found in various clinical groups.They advise to obtain secretion through a bronchoscope whenever the evolution of a lung infection is not conform to criteria established on clinical grounds.