Eligible subjects receiving d4T will be assigned in an open manner to Arm 1, and subjects who have been receiving zidovudine (AZT) will be assigned in a randomized, double blind manner to Arms 2 and 3. Each subject will receive study drug for 48 weeks as follows: Arm 1 - d4T plus ddI, Arm 2 - d4T alone, Arm 3 - d4T plus ddI.