Previous reports (1, 2) have established that the expression of certain distinctive membrane antigen(s) on the surface of Burkitt's lymphoma (BL) and infectious mononucleosis (IM) cells is dependent on the presence of Epstein-Barr virus (EBV) in the cell line. The investigations reported here provide evidence that antibodies directed against EBV antigens, as revealed by the immunofluorescence test on acetone-fixed smears (8), and the membrane reactive antibodies, although often present in the same serum, are nevertheless distinctly different. Absorption of Mutua serum, the standard reference serum for demonstrating membrane antigen(s) on BL and IM cells, with BL cells completely removed anti-membrane activity without significantly affecting the anti-EBV antibody titer. Furthermore, sera were found which contained one type of antibody but not the other. Sera with high anti-membrane but low anti-EBV activity were found among relatives of BL patients. These sera reacted with the membranes of EBV-carrying BL and IM cells in essentially the same way, i.e., against the same spectrum of target cells, as the EBV-positive Mutua serum. They were unable to block the membrane reactivity of FITC-conjugated Mutua serum, however. In some cases they showed weak but incomplete blocking. One such EBV-negative, membrane-positive BL relative serum (Robert) was conjugated with FITC and used for direct staining of BL and IM cells. Again, this conjugate reacted against the same target cell spectrum as a Mutua conjugate, but its reactivity was completely blocked by a number of Burkitt patients' sera, although unconjugated Robert serum did not block the Mutua-conjugate. A number of other membrane-positive BL relative sera also failed to block Mutua, but completely blocked the Robert conjugate. A number of Swedish and African control sera and an isoantiserum gave no blocking against Robert conjugate. It therefore appears that the Mutua conjugate contains at least two antibody specificities against the EBV-determined membrane antigens. One, but not the other, is shared with the antibody specificity present in Robert's serum and a number of other sera from relatives of BL patients.

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