The experiments presented in this study demonstrate that there exist at least two functional epitopes on an I-A molecule that can be recognized by T cell clones. By comparing the abilities of spleen cells from C57BL/6 mice and the congenic I-A mutant line B6.C-H-2bm12 to stimulate alloreactive T cell clones specific for the I-Ab molecule, we have discriminated two sets of clones, those recognizing the I-Ab and I-Abm12 molecule equally well and those able to recognize only the I-Ab molecule. These results imply that the two sets of clones have different receptors for I-A and that they therefore recognize separate epitopes on the I-A molecule. We have similarly been able to separate T cell clones, both alloreactive and L-glutamic acid60-L-alanine30-L-tyrosine10-reactive, specific for the Ab alpha Ak beta hybrid molecule into two groups based on their ability to recognize bm 12 spleen cells. Although the recognition of bm 12 spleen cells by these clones was unexpected since none of them responds to B6 spleen cells, these data again allow us to conclude that these groups of clones have different receptors for the same I-A molecule and therefore that they recognize distinct epitopes on the molecule. Additional studies, in which monoclonal anti-I-A antibodies were used to block the stimulation of T cells by stimulator or antigen-presenting cells, have demonstrated that this blockade can be a steric effect and therefore is not necessarily indicative of direct competition between the antibody and the T cell for the same site on an I-A molecule. Although this study does not reveal the physical nature of an I region-controlled "antigen-restriction site," we can suggest that increasing the number of possible functional Ia restriction sites either through combinatorial association of alpha and beta chains or by using more than one site per molecule will increase the number of configurations the ternary complex of Ia, antigen and T cell receptor(s) can form.