HIV-1 strains, including a molecularly cloned isolate, that had been passaged through different cell types adapted to faster growth in the same cell type and displayed a different host cell tropism. The only change in viral proteins revealed by immunoblot analyses was the molecular size of the envelope glycoprotein gp120 that varied for viruses recovered from the different infected cells. The alteration in size was most likely the result of modification of gp120. Host range differences were also observed for a molecularly cloned HIV-1 strain when passed through the peripheral white blood cells from different individuals. Thus, this phenomenon could have clinical relevance in HIV pathogenesis.