To compare the viral burden and the biological phenotype of HIV-1 isolates obtained from lymphoid node mononuclear cells (LNMC) and peripheral blood mononuclear cells (PBMC) in 11 HIV-infected patients.Viral burden was quantified by cocultivating LNMC and PBMC from HIV-infected patients with PBMC from seronegative donors. For each patient, LNMC and PBMC isolates were characterized in terms of susceptibility to neutralizing antibodies, syncytium-inducing capacity and sensitivity to zidovudine.Our data show that: (1) viral burden was 1.73 log higher in LNMC than PBMC in patients with persistent generalized lymphadenopathy and only 0.37 log higher in patients with AIDS-related complex; (2) five out of 11 LNMC bulk isolates were phenotypically distinct from autologous PBMC isolates; (3) in three patients, the autologous serum neutralized the PBMC isolates but not the LNMC isolates.These results suggest that the relatively high level of HIV-1 replication in lymph nodes may favour the emergence of viruses exhibiting specific phenotypes, including neutralization escape variants. The existence of viral variants in lymphoid tissue at all stages of HIV infection may elucidate certain aspects of the pathogenesis of HIV-1 infection.