We have used flow microfluorimetry to investigate quantitatively the expression of HLA-A, B and C antigens, and beta 2 microglobulin, by cell lines derived from human teratocarcinomas, Although low levels of these cell surface molecules were expressed by all the lines examined, there was no evidence of discrete HLA-A, B, C/beta 2-microglobulin positive and negative subpopulations in any cultures. In contrast, using similar techniques, no murine embryonal carcinoma cell line was found to express the homologous H-2 antigens, It is suggested that human embryonal carcinoma cells may differ from their mouse counterparts by expressing low levels of major histocompatibility antigens