Two monoclonal antibodies, D1-12 and BT 2.2, recognizing two distinct subsets of human Ia molecules, NG1 and NG2, respectively, present in all individuals irrespective of their HLA-DR phenotype, have been used to immunoselect cell variants from the lymphoblastoid cell line Raji. Results showed that, irrespective of the monoclonal antibody used for immunoselection, the cell variants analyzed in this study had lost the expression of both D1-12-and BT 2.2-specific antigenic determinants. Moreover, the expression of antigenic determinants specific for a third family of Ia molecules, the DC-1 subset, were also lost in the cell variants. In contrast, expression of HLA A, B, and C common structures, as recognized by the W6.32 monoclonal antibody, as well as expression of surface immunoglobulins, were not affected. Possible mechanisms inducing such a coordinate loss of expression of several families of human Ia molecules are discussed.