The purpose of the current study is to test whether antibiotic treatment aimed at eradicating Chlamydia infection will reduce the disease activity in MS. The primary outcome measure will be reduction in new enhancing MS lesions on brain MRI. Forty patients will be entered into the trial. To be eligible, patients must have evidence of chlamydia infection in their spinal fluid and enhancing lesions on their pre-randomization MRI scans. Patients who meet these criteria will be randomized to either placebo or antibiotic therapy, and followed for 6 months on treatment.