To confirm results from a previous study in which the combination of thymopentin plus zidovudine ( AZT ), an antiretroviral agent, slowed disease progression in HIV-infected asymptomatic patients. To evaluate the efficacy and safety of thymopentin in HIV-infected asymptomatic patients receiving either monotherapy with AZT, didanosine ( ddI ), or stavudine ( d4T ), or combination antiretroviral therapy with AZT / ddI or AZT / zalcitabine ( ddC ).