With 28 beta-lactam antibiotics the susceptibilities of 60 strains of Legionella spp. (49 Legionella pneumophila and 11 ATCC type strains of other Legionella species) were determined. Agar dilution testing was used on buffered charcoal-yeast extract agar to which 0.1% alpha-ketoglutarate was added. The most active of the penicillins tested were ampicillin (MIC = 0.06-8 mg/l) and temocilin (MIC = 0.125 - 16 mg/l); the most active cephalosporins were ceftazidime (MIC = 0.03 - 0.5 mg/l), HR 810 (MIC = 0.06 - 1 mg/l) and cefoxitin (MIC = 0.125 - 2 mg/l). Of all the drugs tested imipenem had the most pronounced activity (MIC = .0075 - 0.06 mg/l).