12 patients with chronic staphylococcal osteomyelitis were treated with cloxacillin (6 patients) and dicloxacillin (6 patients) in a prospective, comparative study. The drugs were given perorally in the dose 1 g every 4th hour and serum concentrations were followed. 0.5, 1 and 2 hours after administration the serum level of dicloxacillin was about double that of cloxacillin. The lowest serum level of dicloxacillin at the time of administration of the drugs was nearly 3 times as high as the level of cloxacillin. An evaluation of clinical benefit and side-effects revealed no differences between the drugs. The prolonged serum level of dicloxacillin may permit a longer interval between doses than the administration of cloxacillin.