To compare the effect of stavudine (d4T) alone or with zidovudine (AZT) versus didanosine (ddI) alone or with AZT on CD4 counts, HIV RNA levels, and viral load in HIV-infected patients [AS PER AMENDMENT 3/21/97: To compare the effects of d4T alone versus ddI alone versus AZT plus ddI]. To compare the safety of d4T/AZT. AS PER AMENDMENT 3/21/97: To evaluate the pharmacokinetic interactions of AZT and d4T both at an extracellular and intracellular level.