Effect of stage of disease and drug dose on zidovudine susceptibilities of isolates of human immunodeficiency virus.

Ninety-seven isolates of human immunodeficiency virus (HIV) from 73 individuals were assayed for susceptibility to zidovudine (AZT). All isolates from 41 individuals with no known therapy with zidovudine were uniformly susceptible to the drug in vitro. In contrast to isolates from subjects with AIDS or AIDS-related complex, isolates from subjects with fewer signs and symptoms or high CD4 lymphocyte counts developed reduced susceptibility at slower rates and lower levels of resistance. Patients receiving lower doses of zidovudine at both early and late stages of disease did not develop resistance more readily than patients receiving higher doses of drug.