The mechanism by which the rat T cell alloantigen, RT-6.2, is attached to the membrane was investigated. Treatment of rat lymph node and T-hybridoma cells with phosphatidylinositol-specific phospholipase C (PI-PLC) caused a substantial reduction in the amount of RT-6.2 on the cell surface. No significant release of a rat T helper marker (visualized by the mAb W3/25) was observed in response to PI-PLC treatment. This is in sharp contrast to the effects of trypsin, which removes most of the T helper marker but had little effect on RT-6.2. SDS-PAGE analysis of the RT-6.2 released by PI-PLC indicated that the Mr was not significantly changed by this treatment. Phase separation of the released RT-6.2 in Triton X-114 showed that the PI-PLC had converted it from an amphiphilic membrane form to a water-soluble form, apparently by removing its hydrophobic membrane anchoring domain. These results strongly suggest that RT-6.2, in common with Thy-1 and several other cell surface proteins, is anchored in the membrane by the 1,2-diacylglycerol moiety of a covalently attached phosphatidylinositol molecule.