Cefonicid pharmacokinetics in serum and tissue penetration into the lung parenchyma, bronchial mucosa and pleura were studied in 39 patients undergoing lung excision for malignancy. Cefonicid concentrations in serum and tissues samples were assayed at different times after a single 1 g intramuscular administration. The concentrations observed were much higher than the reported minimal inhibitory concentrations for the microorganisms commonly responsible for bronchial and pulmonary infections and therapeutic concentrations were still detectable in the tissues 24 h after dosing. Kinetic findings demonstrated a similar half-life for cefonicid in tissues and in serum. These data provided a further kinetic explanation for the observed clinical efficacy of cefonicid with a single daily dose.