Seven paired HIV-1 isolates from peripheral blood mononuclear cells (PMCs) and cerebrospinal fluid (CSF) of infected subjects at various stages of the disease were studied for their capacity to replicate in continuous cell lines (Molt-3 and U-937 cells) and to induce cytopathic effects "in vitro". Obtained results indicate that paired HIV-1 isolates from PMCs and CSF of the same patient can differ in their replicative activity "in vitro", suggesting that, at least in some cases, CSF isolates may represent a distinct subtype of HIV-1.