SUMMARYSurface antigens of Marek's disease lymphoblastoid cells were studied by indirect immunofluorescence (FA) tests and by cytotoxic antibody tests. The results of FA tests revealed genetic differences between cell lines of different sources with respect to their histocompatibility antigens. JMV and MSB-1 cells shared one of their alloantigens detectable by B blood group antisera.Differences between the cell lines were more pronounced when the respective hyperimmune sera were examined by cytotoxicity tests rather than by FA tests. The results of cross-absorptions of cell line antisera with cells of the different lines suggested that no identical common tumour-specific cell surface antigen was detectable serologically in addition to histocompatibility antigens or other normal cell surface antigens.