Our data show that: Most BL cells express significantly less HLA class I and II than the corresponding LCLs. Lymphoblastoid cell lines have a much higher HLA class I and II density than normal peripheral blood B cells. There are qualitative differences in class I and II expression in most BL/LCL pairs. These qualitative differences are not caused by EBV infection. They might be due to a selective downregulation of HLA specifities by c-myc activation. We conclude that these modifications of HLA expression may well play a role in the reduced specific immune recognition of Burkitt's lymphoma cells.