In vivo infection of monocytes/macrophages by the human immunodeficiency virus (HIV) has been investigated in many studies since these cells were suggested to provide a reservoir for the virus. In this study, we wanted to find out whether HIV provirus could be detected in circulating monocytes and whether it could be compared with the provirus found in T lymphocytes (T-Ly). Twenty-one seropositive subjects were studied. The amplification method (PCR) was used with three different primer pairs (in gag, env, and long terminal repeat regions of the viral genome) to detect the HIV-1 genome in monocytes and T-Ly separated by an immunomagnetic isolation technique. Of 21 monocyte samples, 13 (61.9%) were positive with at least one primer pair. Furthermore, the provirus harboured in 9 of those 13 monocyte-positive samples differed, with respect to pattern of primer response, from the provirus found in T-Ly. When comparing primer responses of monocytes and T-Ly, most of the differences were found to have occurred with the env primers (8 of 9 cases). Dilution experiments with the 8 E5 cell line revealed that 9 of 12 T-Ly contained 15-150 HIV DNA copies per 150,000 cells while 8 of 11 positive monocytes contained less than 15 copies. However, monocyte samples from two asymptomatic individuals and an AIDS patient showed high levels of HIV DNA, comparable to those obtained in T-Ly. Finally, it was also found that the monocyte-positive subjects were more immunosuppressed than the negative ones, as shown by the total CD4 count of both groups (means of 269 T4/mm3 and 573 T4/mm3, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)