All patients diagnosed with MAC and who will initiate at least a 2 drug clarithromycin containing MAC treatment regimen will be eligible for participation. Blood and urine will be obtained from each patient at the following timepoints: Pre-Entry (within 7 days prior to study entry), week 4, and week 8. Sites will process and ship specific samples to Case Western Reserve University (CWRU). Various assays and analyses will be performed by CWRU. NOTE: Patients will receive no treatment on this study, however, all patients must be receiving at least a 2 drug clarithromycin containing treatment regimen for MAC either as part of participation in other studies or as prescribed by the subject's health care provider.