Expression of HLA class I antigens on the cell surface requires the association of heavy chain, endogenous peptide and beta 2-microglobulin (beta 2m). We have studied the expression of free class I heavy chain and beta 2m in cryostatic sections of 51 HLA class I negative human tumours of different origin. Selective low expression of beta 2m was observed only on the 9 colorectal adenocarcinomas. In contrast, simultaneous under-expression of heavy chain and beta 2m was detected in the rest of the tumours studied. Our results suggest that the accumulation in the cytoplasm of HLA heavy chain in HLA negative colorectal carcinomas is a particular mechanism that cannot be applied to all HLA negative tumours.