Sputum and bronchial biopsies and smears were obtained from 221 patients who had undergone partial pneumonectomy without preceding antibiotic therapy. The results of 343 sputum examinations and of 126 bacteriological examinations of biopsies and smears were evaluated. Pathogenic bacteria were demonstrated in about 50% of the sputa and in about 25% of the biopsies and swabs. Anaerobic micro-organisms were extremely rare. Treatment was with tetracycline. Before antibiotic therapy Haemophilus influenzae, staphylococci and E. coli predominated, afterwards there were practically only staphylococci and Esch. coli. The incidence of primary resistance to the usual antibiotics was high, especially in respect of gram-negative organisms. The difference in incidence between primary and secondary resistance was one of degree. The least effective agents were ampicillin and amoxicillin, the most active were the aminoglycosides. Erycin proved satisfactory in infections with grampositive organisms while the cephalosporins were active against both gram-positive and gram-negative bacteria. The development of resistance and the clinical picture suggest that "eradication" of the infection during the postoperative stage is not advisable as it causes the selection of more or less drug-resistant organisms. A more satisfactory approach is gradually to reduce the infection until the immediate postoperative stage is over. Macroscopic and microscopic examination of the sputum is important, as the choice of the appropriate antibiotic is determined by the result of gram-staining.