Lymphocyte function-associated antigen 3 (LFA-3) is a widely distributed cell surface glycoprotein that binds to the T lymphocyte CD2 surface glycoprotein. This interaction mediates CTL-target cell conjugate formation and adhesion of thymocytes to thymic epithelial cells. CD2 is also the E rosette receptor of T lymphocytes and mediates rosetting with autologous E by binding to LFA-3. We describe deficient expression of LFA-3 on E from paroxysmal nocturnal hemoglobinuria (PNH) patients. PNH is an acquired defect affecting phosphatidylinositol-anchored membrane proteins, of which decay-accelerating factor (DAF) is most important in the clinical symptoms of PNH. LFA-3-negative, weakly positive, and positive populations were found among PNH E. There was a good correlation with DAF deficiency. PNH E exhibited decreased binding of 125I-CD2 and rosetting with a human T lymphoma cell line. PNH E readily incorporated purified LFA-3, restoring LFA-3 expression and the CD2 binding and rosetting activity to normal levels. The expression of DAF was not restored after the incorporation of purified LFA-3 into PNH E, showing that LFA-3 and DAF are different molecules. Phosphatidylinositol-specific phospholipase C (PIPLC) treatment of a B lymphoma cell line released 35% of the cell surface LFA-3 and 62% of DAF. LFA-3 on E was resistant to PIPLC. However, when LFA-3 purified from human E was reconstituted in sheep E or human E and subjected to PIPLC treatment, 40-50% of LFA-3 was released from the cell membrane. The results show that LFA-3 is attached to the cell membrane by a phosphatidylinositol glycolipid moiety, and confirm previous findings (37-41) that LFA-3 is a cell adhesion molecule that mediates adhesion by interacting with CD2 antigen.