To determine the time course of development of resistance to atevirdine mesylate ( U-87201E ) in patients with HIV isolates showing in vitro resistance to zidovudine ( AZT ). To determine the genotype changes in HIV reverse transcriptase associated with in vitro AZT resistance to U-87201E. To determine the genotype and phenotype effects of treatment with a nondideoxynucleoside agent on the alterations of the HIV-1 population associated with in vitro AZT resistance. To determine whether serial passage of patient pre-drug HIV isolates in the presence of U-87201E will generate the resistant mutants that may subsequently emerge in the patients.