Patients are randomized in a blinded fashion to receive AZT or placebo in combination with open-label d4T or ddI for up to 48 weeks. AS PER AMENDMENT 3/21/97: The study is now composed of three arms: open-label d4T versus open-label ddI plus blinded AZT placebo versus blinded AZT plus open-label ddI. Patients originally assigned to the d4T + AZT arm, which was closed 10/96, will be given the option of discontinuing AZT and remaining on d4T monotherapy or discontinuing all study drugs. In addition, all study participants will be asked to participate in a pharmacology substudy.