A novel mechanism of protein attachment to cell membranes involving the covalent linkage of the protein through an oligosaccharide to phosphatidylinositol has recently been defined. Many proteins that are anchored to the cell membrane by this mechanism can be released by treatment with phosphatidylinositol-specific phospholipase C (PI-PLC). Monoclonal antibodies are useful as probes in the study of the roles of cell-surface components in neutrophil function. Many monoclonal antibodies that bind to human neutrophils react with the oligosaccharide lacto-N-fucopentaose III (CD15 antibodies). Human neutrophil surface proteins identified by 125I surface-labeling using lactoperoxidase were examined for PI-PLC sensitivity, to identify the major surface proteins of human neutrophils that are anchored by a glycosyl-phosphatidylinositol linkage. The major surface-labeled protein identified by lactoperoxidase-catalyzed iodination was a approximately 68-90-kDa protein. Three major surface proteins identified by 125I-surface labeling of approximately 68-90, 57, and 33-kDa were released by PI-PLC treatment. Immunoprecipitation and subsequent gel electrophoresis of proteins from neutrophils labeled with 125I revealed a previously unidentified 98-115-kDa protein specifically reactive with CD15 antibodies that was released from the cell by treatment with PI-PLC. The roles of these proteins in neutrophil function remain to be determined.