The fund normally invests at least 80% of its net assets in municipal bonds and debentures, the income from which is exempt from federal income tax (including the federal alternative minimum tax (AMT)). It invests principally in investment grade municipal obligations, rated at the time of purchase by at least one major rating agency, but may invest up to 30% of its net assets in non-investment grade municipal obligations (sometimes referred to as "high yield" or "junk" bonds).