A total of 950 MLC combinations were set up out of which 259 were classified as low MLC response. An attempt was made to predict which of the combinations would give low response by matching the B-lymphocyte specificities and the HLA-A and -B antigens. A statistically significant prediction of a low response was noted for the B-lymphocyte specificities as well as for HLA-A and -B antigens. The fraction of the low response, which was correctly found, was, however, greater with the B-cell specificities than the HLA-A and -B locus antigens. It is concluded that the B-lymphocyte specificities may be either identical or closely linked to the DW specificities. To what extent they are associated with the strong transplantation antigens of man remains to be determined.