This trial is the final important step in determining whether minocycline improves the course of ALS. The principle objective of this clinical trial is to determine whether minocycline slows disease progression and helps maintain function in patients with ALS. This multi-center placebo-controlled study will select patients early in the course of ALS, when a neuroprotective therapy may be most beneficial. The study will measure change in function (as detected by ALSFRS-R scores), strength, pulmonary function, survival, and quality of life. Participants will undergo monthly outpatient evaluations and analysis of laboratory and adverse events. This is a 13-month study.