The interaction between glucocorticoids and antibiotics in the treatment of brain abscesses has been examined in rats. After stereotactic inoculation of Bacteroides melaninogenicus in the right frontal lobe to induce abscess formation, the animals were divided into two groups those treated with antibiotics alone and those treated with both antibiotics and corticosteroids. The antibiotics used, injected 4 days after the bacterial inoculation, were benzylpenicillin, 3H-lymecycline and 14C-metronidazole. The left hemisphere in each animal served as control. Finally a group of animals were inoculated in the right hemisphere with saline and served as controls. Antibiotic assays showed that the use of steroids reduced antibiotic concentration in uninfected and infected tissue with significant reduction in the infected tissue of animals treated with benzylpenicillin and lymecycline. In animals treated with metronidazole, no significant alteration of concentration levels was found. The use of steroids in the treatment of brain abscesses seems to only partially inhibit adequate concentrations of antibiotics in brain tissue dependent upon the antibiotics used.