To study the function of side chain oligosaccharides of the cell-surface lipophosphoglycan (LPG), mutagenized Leishmania major defective in side chain biosynthesis were negatively selected by agglutination with the monoclonal antibody WIC79.3, which recognizes the galactose-containing side chains of L. major LPG. One such mutant, called Spock, lacked the ability to bind significantly to midguts of the natural L. major vector, Phlebotomus papatasi, and to maintain infection in the sand fly after excretion of the digested bloodmeal. Biochemical characterization of Spock LPG revealed its structural similarity to the LPG of Leishmania donovani, a species whose inability to bind to and maintain infections in P. papatasi midguts has been strongly correlated with the expression of a surface LPG lacking galactose-terminated oligosaccharide side chains. An in vitro galactosyltransferase assay using wild-type or Spock membranes was used to determine that the defect in Spock LPG biosynthesis is a result of defective beta1,3-galactosyltransferase activity as opposed to a modification of LPG, which would prevent it from serving as a competent substrate for galactose addition. The results of these experiments show that Spock lacks the beta1, 3-galactosyltransferase for side chain addition and that the LPG side chains are required for L. major to bind to and to produce transmissible infection in P. papatasi.