Since Giardia lamblia trophozoites are exposed to high concentrations of fatty acids in their human small intestinal milieu, we determined the pattern of incorporation of [3H]palmitic acid and myristic acid into G. lamblia proteins. The pattern of fatty acylation was unusually simple since greater than 90% of the Giardia protein biosynthetically labeled with either [3H]palmitate or myristate migrated at approximately 49 kDa (GP49) in reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis during both growth and differentiation. GP49, which partitions into the Triton X-114 detergent phase, is localized on the cell surface since it is 125I-surface-labeled. GP49 was also biosynthetically labeled with [14C]ethanolamine and [3H]myoinositol, suggesting that it has a glycosylphosphatidylinositol (GPI) anchor. Moreover, phospholipase A2 (PLA2) or mild alkaline treatment released free fatty acids, indicating a diacylglycerol moiety with ester linkages. Finally, a 3H- and 14C-labeled species was released by nitrous acid deamination from [14C]palmitate- and [3H]myoinositol-labeled GP49. The GPI anchor of GP49 is unusual, however, because purified GP49 was cleaved by Bacillus cereus phosphatidylinositol (PI)-specific PLC, but not by Staphylococcus aureus PI-PLC, or plasma PLD, and did not react with antibody against the variant surface glycoprotein cross-reactive determinant. Moreover, the double-labeled deaminated GP49 anchor migrated faster than authentic PI in TLC and produced [3H]glycerophosphoinositol after deacylation. In contrast to the variable cysteine-rich G. lamblia surface antigens described previously, GP49 was identified in Western blots of every isolate tested, as well as in subclones of a single isolate which differ in expression of a major cysteine-rich 85/66-kDa surface antigen, which does not appear to be GPI-anchored. These observations suggest that GP49, the first common surface antigen to be described in G. lamblia, may play an important role in the interaction of this parasite with its environment.