To validate that the alteration of codon 215 of reverse transcriptase in plasma virus precedes the increase in viral burden as measured in the peripheral blood and the decline in CD4 count that have been observed in association with clinical failure on zidovudine (AZT). To determine whether alternative regimens of antiretroviral agents alter the course of viral burden as measured in the peripheral blood and CD4 changes in patients with HIV infection. To obtain further data on the safety and immunologic and virologic response to AZT/didanosine/nevirapine.