A panel of colorectal carcinomas has been examined immunohistologically with mAb specific for allodeterminants of HLA-A or HLA-B, and with mAb reactive with HLA-A,B,C framework determinants or beta 2-microglobulin. Out of 85 carcinomas, 5 cases were completely negative for all class I Ag in the tumor cell population. Fifteen cases exhibited a differential expression of HLA-A and HLA-B products in all tumor cells or in a subset. Such tumor cells showed strong staining with monomorphic HLA antibodies but failed to stain with certain allospecific mAb. Negativity of tumor cells was scored only when normal stromal cells within the tumor mass were clearly labeled, indicating expression of the particular allodeterminant(s) in the given tissue. Four carcinomas showed a selective loss of HLA-A2 or a non-A2 HLA-A allospecificity in all tumor cells or a major subset, whereas HLA-B Ag were expressed. Seven cases showed a selective loss of the HLA-Bw6 superspecificity in the tumor cell population; HLA-Bw4 and HLA-A Ag were present. Three cases exhibited a combined loss of HLA-A2 and HLA-Bw6 Ag, with non-A2 HLA-A and HLA-Bw4 Ag being expressed. A further case was HLA-Bw6/Bw4 negative in a major tumor cell subset and HLA-A negative in the complementary minor subset. The results show that a considerable proportion of colorectal carcinomas is heterogeneous with regard to HLA-A and HLA-B expression. The reasons for the appearance of tumor subsets with complete loss of class I Ag or selective loss of only HLA-A or HLA-B Ag are not clear, but it is conceivable that some of them arose because they have escaped immunoselection.