Our recent studies have indicated that galactosyl ceramide (GalCer) or sulfatide (sul) may serve as an alternate receptor for human immunodeficiency virus (HIV) in neural cells. In this paper, we describe the mapping of GalCer/sul binding region of HIV env glycoprotein gp120. Deglycosylated gp120 binds to GalCer, suggesting that the amino acids of glycoprotein gp120 and not the carbohydrates are responsible for the observed binding. Specific regions of gp120 responsible for the binding were analyzed by using varying-length truncations of gp120 expressed in Escherichia coli and vaccinia virus. Purified recombinant gp120 containing amino acids 200-295 of gp120 bind to GalCer/sul, whereas recombinant env proteins that deleted this region did not bind. These recombinant proteins also bind to SK-N-MC-derived neuroblastoma cells, the binding of which is inhibited by anti-GalCer. In addition, 125I-labeled gp120 binding to GalCer is inhibited by these proteins. Studies using lysates containing truncated gp120 expressed in vaccinia virus also gave similar results. By eliminating the overlapping regions that do not bind, we conclude that the amino acids responsible for GalCer/sul binding reside between amino acids 206 and 275. The significance of this mapping is discussed in relation to the neurotropism of HIV.