Substantial amounts of mezlocillin are rapidly distributed in a wide range of tissues. After intravenous administration of doses ranging form 2 to 5 g, concentrations therapeutically active against most sensitive organisms were obtained in bone, prostate, skin, muscle and liver, as well as in wound exudates, pleural exudates, sputum and bronchial secretions. Mezlocillin did not easily cross the blood-brain barrier with normal meninges, but CSF concentrations when the meninges were inflamed. The antibiotic also crossed the placenta.