The emergence of zidovudine (3'-azido-2',3'-deoxythymidine)-resistant strains of human immunodeficiency virus type 1 (HIV-1) from AIDS patients treated with zidovudine has been linked to six amino acid substitutions localized within the viral polymerase gene. Here, in 2 patients, three resistance mutations were detected by polymerase chain reaction amplification of HIV-1 polymerase (reverse transcriptase) sequences from cultures of patient plasma only and not from the same patients' uncultured leukocytes. The differences in distribution of the mutant genotypes from the two sources were highly significant. Both plasma- and peripheral blood mononuclear cell (PBMC)-derived virus (and/or RNA genomes) should be studied in additional subjects to confirm the hypothesis raised by these data that zidovudine-resistant virus may be more frequent in plasma than in uncultured PBMC.