The immunohistological characteristics of infiltrating lymphoid cells and the expression of human leucocyte antigens class I and II (HLA-ABC and HLA-DR, respectively) were studied in 50 pre-treatment nasopharyngeal carcinomas. The majority of lymphoid cells were activated lymphocytes expressing thymocyte OKT10 marker. CD4+ cells (T-helper/inducer) out-numbered CD8+ cells (T-suppressor/cytotoxic) by at least two- to four-fold. CD22+ cells (pan-B lymphocytes) were scanty in the peri-tumoral areas and were absent in 29 out of 50 biopsies. A moderate number of cells expressing CD15 (monocytes/macrophages) were also detected. CD16+ cells (natural killer cells) were found to be sparse or absent. Expression of HLA class I and II antigens on the tumor cells in 35 biopsies was variable. HLA-ABC staining was intense in 6, reduced in 13 and partially lost in 16, whereas staining of HLA DR was intense in 7, reduced in 11 and partially lost in 17. Full expression of both antigens was demonstrable in only 2 biopsy samples. The expression of HLA antigens in the tumour had no relationship to the type or degree of lymphocytic infiltration or staging of the tumour.