Summary— Biosynthetic labelling experiments performed on P primaurelia strain 156, expressing the temperature‐specific G surface antigen, 156G SAg, demonstrated that the purified 156G SAg contained the components characteristic of a GPI‐anchor. [3h]ethanolamine, [3h]myo‐inositol, [32p]phosphoric acid and [3h]myristic acid could all be incorporated into the surface antigen. Myristic acid labelling was lost after treatment in vitro with Bacillus thuringiensis phosphatidylinositol‐specific phospholipase C (PI‐PLC). After complete digestion by pronase, a fragment containing the intact GPI‐anchor of 156G surface antigen was isolated. This fragment was shown to be hydrophobic and glycosylated and to possess an epitope found specifically in the GPI component of GPI‐anchored proteins. The role of the GPI‐tail in anchoring the 156G surface antigen into the membrane was assessed by determining that purified 156G molecules with the GPI‐anchor could be incorporated into lipid vesicles and red cell ghosts whereas the 156G molecules lacking the GPI‐anchor, as result of treatment with B thuringiensis PI‐PLC, could not. It has also been shown that the membrane‐bound form and the soluble form, obtained after cleavage of the 156G SAg lipid moiety either by an endogenous PI‐PLC or by a bacterial PI‐PLC, displayed identical circular dichroic spectra.