In a previous study, we showed that the three hitherto serologically indistinguishable HLA-D specificities associated with HLA-DR4, HLA- DYT , HLA- DKT2 , and HLA-Dw4 can be distinguished on the basis of their reactivity with two distinct Ia-like-specific monoclonal antibodies, HU-18 and HU-23. In this study, we attempted to identify and characterize Ia-like molecules recognized by HU-18 and HU-23 on a molecular level because Ia subsets (HLA-DR, MB, MT, or SB) identified by them remained unknown. The results of sequential coprecipitation assays and two-dimensional gel analyses showed that both HU-18 and HU-23 recognize antigenic determinants borne on MB3 but not on HLA-DRw6.2 molecules. Because the two monoclonal antibodies, specific for determinants carried on MB3 molecules, show distinct reactivity against homozygous typing cells defining HLA- DYT , HLA- DKT2 , and HLA-Dw4, all of which share DR4- MB3 , the data indicate that these three HLA-D clusters associated with HLA-DR4 possess distinct MB3 molecules, suggesting the existence of polymorphism in MB3 antigens.