The fund invests at least 80% of its net assets (including borrowings for investment purposes) in municipal obligations that are exempt from regular federal income tax. It may invest without limit in obligations the income from which is subject to the federal alternative minimum tax. At least 65% of net assets normally will be invested in municipal obligations rated at least investment grade at the time of investment. The fund will not invest more than 10% of its net assets in obligations rated below B.