In order to investigate the distribution of epitopes recognized by T-cell clones directed against HLA class II products, bulk primed cell populations were generated using cells matched for class I determinants but disparate for class II determinants. Cells were cloned by single cell deposition (FACS IV) or limiting dilution (1 cell/3 wells), and assayed for proliferative and cytolytic function with panels of well-characterized cells. All cytolytic clones generated from an anti-DR4/Dw4/DQw3 priming combination or an anti-DR2/Dw2/DQw1 priming combination lysed essentially all targets sharing the same Dw type as the sensitizing cell. In some cases, other targets were also lysed. For instance, some clones were lytic to targets bearing the same DR antigen but another Dw subtype including a few clones lytic to virtually all cells carrying that DR specificity. An occasional target cell expressing a different DR antigen from the sensitizing cell was also lysed by these clones, in some cases to the same extent of lysis seen on the specific target. Monoclonal antibody inhibition studies identified three groups of clones: the DQ directed clones and clones apparently directed at more than one DR product. However, the number of molecules detected for each haplotype remains to be investigated. Our data indicate that determinants detected on both DR and DQ products are associated with the Dw type of the sensitizing cell showing that there is polymorphism recognized by T cells on both DR and DQ that is subtypic to the serologically defined specificities. Thus, it appears that the bulk T-cell response is a composite of individual clones recognizing distinct determinants on these class II molecules. The implications of these findings for studies of HLA restricted recognition are discussed.