The HLA-D region encodes for several serologically defined systems, including DR, MB, and MT. The antigens of MB and MT are strongly associated with two or more DR specificities. The purpose of this study was to determine the role of MB and MT antigens in lymphocyte alloactivation. A soft agar colony assay was used to generate alloreactive lymphocyte clones primed in mixed leukocyte culture against a stimulator who typed as HLA-DR4,-;MB3,-;MT3,-. In secondary primed lymphocyte typing (PLT) assays, several clones were identified with PLT specificities strongly associated with DR4, MB3, or MT3. The data suggest that HLA-D controls different lymphocyte-activating determinants associated with the serologically defined DR, MB, or MT antigens.