Patients, ages 18 to 65, with a diagnosis of bipolar depression (without psychotic features), will be randomized to double-blind treatment to receive either ceftriaxone (1-4 g/day) or placebo intravenously for a period of 6 weeks. Acute efficacy will be determined by demonstrating a greater response rate using specified criteria. Approximately 86 patients with bipolar depression will be enrolled in the study.