We have previously shown that the cell line 6D5(451) chronically infected with the HIV-1 isolate HTLV-III(451), secretes the HIV-1 envelope glycoproteins gp120 and gp160 in the extracellular medium. The HTLV-III(451) gp120 and gp160 were purified by sequential affinity chromatographic steps using a monoclonal antibody to HIV-1 gp41 and an anti-HIV-1-positive human serum. Amino acid sequence analysis of gp120 and gp160 showed the loss of the signal peptide. Digestion of the purified gp120 and gp160 with endoglycosidases revealed that both proteins are heavily glycosylated and contain complex carbohydrates, in contrast to the intracellular form of gp160 which has been shown to contain mannose-rich immature sugars. Competitive binding analysis showed that while both gp120 and gp160 bind CD4, the affinity of gp160 was five times lower than that of gp120. Both gp120 and gp160 inhibited syncytia formation by HIV-1-infected cells when mixed with CD4+ cells. Furthermore, both gp120 and gp160 had strong mitogenic effects on the T cells from HIV-1-infected gibbons but not on cells from uninfected gibbons.