To further delineate the functional defects of bm12 mice in antigen presentation, we analyzed antigen-specific T lymphocyte clones derived from B6 or bm12 for their ability to recognize antigens presented on either B6 or bm12 APC. Both complex antigens, such as PPD and GAT, and restricted antigens, such as beef insulin, were used. Our results indicate that for complex antigens, more than 50% of the B6 T lymphocyte clones recognized antigens only if presented by B6 APC, whereas the rest could not discriminate B6 from bm12 APC. Similarly, bm12 T lymphocyte clones responding to complex antigens could be divided into two groups depending on the sources of the APC. We have also isolated B6 T lymphocyte clones specific for the more restricted antigen, beef insulin, to which bm12 failed to respond. All B6 T lymphocyte clones could be stimulated only with B6 APC and not with bm12 APC. These data are consistent with the notion that there are antigen-specific association sites on the Ia molecule, and that complex antigens have more than one such association site. Furthermore, these studies demonstrate that both the gain and loss determinants associated with the bm12 mutation are recognized by a significant number of bm12 and B6 antigen-specific T lymphocyte clones, respectively, thus defining the importance of this region of the A beta polypeptide chain in antigen presentation.