Although an x-ray crystal structure of lactose permease (LacY) has been presented with bound galactopyranoside, neither the sugar nor the residues ligating the sugar can be identified with precision at ~3.5 Å. Therefore additional evidence is important for identifying side chains likely to be involved in binding. Based on a clue from site-directed alkylation suggesting that Asn272, Gly268 and Val264 on one face of helix VIII might participate in galactoside binding, molecular dynamics simulations were carried out initially. The simulations indicate that Asn272 (helix VIII) is sufficiently close to the galactopyranosyl ring of a docked lactose analogue to play an important role in binding, while the backbone at Gly268 may be involved, and Val264 does not interact with bound sugar. When the three side chains are subjected to site-directed mutagenesis, with the sole exception of mutant Asn272Gln, various other replacements for Asn272 either markedly decrease affinity for substrate (i.e., high KD) or abolish binding altogether. However, mutant Gly268Ala exhibits a moderate eight-fold decrease in affinity, and binding by mutant Val264Ala is affected only minimally. Thus, Asn272 and possibly Gly268 may comprise additional components of the galactoside-binding site in LacY.