The extent of latent HIV-1 infection in blood T cells and monocytes of 23 seropositive individuals was examined using DNA amplification (PCR) of HIV-1 sequences. Amplified DNA was found in at least one cell type in all seropositives tested, including 13 asymptomatic, 5 ARC, and 5 AIDS patients. Amplification with two or more primer sets from the gag, env, LTR occurred in 21 (91%) patients' T cells and 17 (74%) patients' monocytes. However, amplification with the LTR primers in monocytes was uncommon. Among four patients tested, amplified DNA continued to be detected after a greater than one thousand-fold dilution (less than 500 cells) of both T cell and monocyte lysates. Repeat analysis after 7-9 mo in five seropositives yielded similar findings in T cells and monocytes, but some variation in the efficacy of amplification with individual primers occurred. There was no difference in those 10 patients who were taking AZT, compared to those who were untreated. Our results indicate that a fraction (less than 1%) of both T cells and monocytes in blood carry a latent infection in all stages of HIV-1 disease and can serve as reservoirs throughout AZT therapy.