A pictorial map of the lactose synthase (galactosyl transferase) acceptor binding site has been formulated from this and published studies on substrate analogs and inhibitors. The basic requirements are a pyranose, thiopyranose or inositol ring structure and equatorial substituents (if any) at C-2, C-3, C-4, and C-5. The aglycone (at C-1) may be either alpha or beta-, but alpha- is somewhat preferred. In the absence of alpha-lactalbumin galactosyl transferase will accept long chain 2-N-acyl substituents on the glucosamine (GlcNH2) structure. An equatorial amino or N-acetyl substituent (e.g. mannosamine, N-acetylmannosamine) is also a suitable acceptor in the absence of alpha-lactalbumin since both N-acetylglucosamine and N-acetylmannosamine have complementary binding loci for the N-acyl moiety. The aglycone moiety must be equatorial (beta-configuration). However, upon alpha-lactalbumin binding the aglycone specificity allows for axial (alpha-configuration) as well as equatorial substituents. Furthermore, the 2-N-acyl substituent binding locus is blocked beyond a 2-N-hexanoyl group. It is suggested that alpha-lactalbumin binds to a hydrophobic site some distance from the C-2 group.