The 65 kDa C8-binding protein or homologous restriction factor (C8bp/HRF) protects cells from complement (C)-mediated lysis by binding to C8 and abrogating lytic channel formation. Human C8bp/HRF is shown here to be immunologically related to human C8 and C9 and to murine lymphocyte poreforming protein (PFP, perforin). Polyclonal antibodies raised against purified C8, C9 and perforin react with C8bp/HRF. The antigenic epitopes shared by these four proteins are limited to cysteine-rich or disultide bridge-masked domains. Only complement proteins or perforin that have been disulfide-reduced elicit the production of cross-reactive antibodies when used as immunogens. Analogously, only C8bp/HRF that has been disulfide-reduced reacts with these antibodies. These results suggest that C8bp/HRF may belong to the complement/perforin supergene family. The function of homologous domains shared by these four proteins remains to be elucidated.