This is a phase II, randomized, four-arm study of the virologic and immunologic effects of D4T versus ddi versus zdv plus D4T versus zdv plus ddi in HIV-infected subjects with cd4 cell counts between 300-600 mm3 and greater than 12 weeks zdv experience. Treatment with ddi or D4T was open label. Treatment with zdv was blinded. The primary endpoints are short term(measurements within first 16 weeks) and long term (48 week) changes in (1) RNA copy number, (2) cd4 cell counts and (3) proportion of subjects with laboratory abnormalities and proportion of subjects with adverse signs and symptoms.