The abundant surface glycolipid, lipophosphoglycan (LPG), of Leishmania promastigotes is composed of phosphosaccharide repeating units linked via a phosphosaccharide core to a conserved lyso alkylphosphatidylinositol membrane anchor. It is shown in this paper that monoclonal antibodies (mAbs) directed against LPG also react with an acid phosphatase secreted by L. mexicana promastigotes. Acid phosphatase purified by column chromatography (apparent Mr = 100,000) reacts in immunoblots with the anti-LPG mAb AP3 and another mAb, L3.13, which does not recognize LPG. mAb L3.13 was used to purify the enzyme by affinity chromatography. The resulting glycoprotein has the same molecular weight and binds AP3 on immunoblots. The secreted phosphatase is non-covalently associated with a high molecular weight, galactose-containing glycan or proteoglycan that is recognized by both AP3 and L3.13. In addition to acid phosphatase, other parasite proteins appear to be modified by LPG epitopes.