In vitro activity of cefuroxime, a new cephalosporin stable to bacterial beta-lactamases, was compared with that of cefalothin and other cephalosporins by serial dilution test in more than 600 bacterial strains. Cefuroxime was more active than cefalothin on most strains of Gram negative bacilli (except Salmonella species) and also on most strains of cefalothin-resistant bacteria. In comparison to cefalothin, cefoxitin and cefamandol, cefuroxime exerted the strongest activity on meningococci, streptococci of group A and B and also on Citrobacter freundii. It was as active as cefamandol and more active than cefalothin and cefoxitin on Haemophilus influenzae (also in ampicillin-resistant strains). Pharmacokinetic studies were performed in 10 healthy adult volunteers after i.v. injection of 0.75 g, 1 g, and 1.5 g cefuroxime and of 1 g cefalothin. Cefuroxime was superior to cefalothin by slower renal excretion, longer half-life, lesser or no metabolization and better tissue penetration. Cefuroxime is well tolerated and should be administered in adequate doses corresponding to the severity of the disease and the susceptibility of the causative agent.