Fixation of complement-related protein fragments occurs following treatment of Raji, a CR2-positive lymphoblastoma cell line, with NHS but not with hi-S.From the immunoblot developed by autoradiography of cells pretreated with OKB7, an anti-CR2 moAbandsubsequently treated with I125NHS, two proteins of 60 and 74 kD were visible. These proteins should be cleavage products, components of the cell surface C3c network, poles of attraction for low m.w. molecules and thus could provide acceptor sites for the fixation of complement-related protein fragments. Indirect evidence would indicate that free C3doras a component of a complement protein, could be the delivery system of low m.w. molecules to highlight the surface Ag/Ab interaction sites.Flow cytometric data would suggest that Raji cells secrete or shed proteins that act as autocrine factors. They are involved in the coating and disclosing processes of C3c determinants, the detection of which is strongly inhibited by cell treatment with NHS but not by hi-S