The present study was undertaken in order to investigate the penetration of cefonicid, a long-acting parenteral cephalosporin, with enhanced activity against most gram-positive and gram-negative pathogens, into human lung tissue and lymph nodes in patients undergoing open thoracotomy. Samples of lung tissue, lymph nodes and serum were obtained at various times after a single intramuscular dose of 1 g. The concentration of cefonicid was assayed by an agar diffusion method with Bacillus subtilis used as the test organism. The mean concentrations of cefonicid in serum at 2, 4, 8, 12 and 24 h after the injection were 91.5, 66.1, 35.7, 21.8 and 2.9 micrograms/ml, respectively. The mean levels of cefonicid into the hilar lymph nodes at the same times were 22.3, 18.7, 12.0, 6.9 and 1.5 micrograms/ml, respectively, while its concentrations in lung tissue were lower than those in lung lymph nodes up to the 12th hour (12.1, 14.6, 7.8, 5.4 and 1.9 micrograms/ml, respectively). Our results show that cefonicid was well distributed in interstitial fluid from which pulmonary lymph is formed and that its concentrations in lung tissue and lymph nodes were sufficient to inhibit most pathogens involved in respiratory tract infections. This finding was considered important, because it demonstrated that the high binding by plasma protein of cefonicid did not prevent it from entering lung tissue and fluids in useful quantities.