Flucloxacillin is a new isoxazolyl penicillin active in vitro against gram-positive cocci. of 190 clinical isolates of Group A β-hemolytic streptococci, pneumococci and both penicillin G-sensitive and penicillin G-resistant Staphylococcus aureus, were inhibited by 0.78 μg per milliliter or less of isoxazolyl penicillin. Mean peak serum levels in normal volunteers following the oral administration of 250 mg. and 500 mg. of flucloxacillin were 5.7 and 11.4 μg per milliliter, respectively. The mean peak serum level was 11.8 μg per milliliter following the intramuscular administration of 250 mg. of flucloxacillin. Serum levels with flucloxacillin were lower than those obtained with dicloxacillin, but greater than those with cloxacillin. Flucloxacillin was 92.6 per cent bound to human serum proteins and provided higher serum levels of free drug than the other isoxazolyl penicillins. © 1972.