Pgp-1 is a major cell-surface phosphoglycoprotein (relative mass approximately 95 000) that has been identified in mouse and human cell lines and tissues (Isacke et al. 1986, Trowbridge et al. 1982, Hughes et al. 1983). Although monoclonal antibodies (mAb) that recognize this glycoprotein were initially raised against plasma membranes from NIH/3T3 fibroblast cells (Hughes and August 1981), its designation of Pgp-1 (phagocyte glycoprotein-1) was based on its marked abundance on macrophages, monocytes, and polymorphonuclear cells (Colombatti et al. 1982). Despite that the function of Pgp-1 is unknown, the mouse homolog is a useful differentiation antigen in that it is expressed on prothymocytes in the bone marrow (Trowbridge et al. 1982) and on a proportion of mouse thymocytes, including cells thought to be the immediate descendants of bone marrow prothymocytes (Lesley et al. 1985a, Hyman et al. 1986). Many cells found in an adult mouse thymocyte subpopulation enriched in Pgp-1 Γ· cells contain an unrearranged Tcell antigen receptor B-chain gene (Trowbridge et al. 1985). Further, Pgp-1 is expressed on more than 90% of fetal thymocytes on days 13 and 14 of gestation before the fraction of Pgp-1 Γ· cells declines to adult levels shortly before birth (Lesley et al. 1985b). In contrast to the mouse, 50-60 % of human thymocytes express Pgp-1. The Pgp-1 Γ· subpopulation in the human comprises primarily medullary thymocytes, although some Pgp-1 Γ· cells scattered throughout the cortex can be detected by immunofluorescence staining (Isacke et al. 1986).
p85 is a human glycoprotein with an approximate relative mass of 85 000 which was initially isolated from human chronic lymphocytic leukemia cells using mAb 50B4 and 50E6 (Letarte et al. 1985). More recently, the p85 glycoprotein was shown to carry all the antigenic deter-