The fine specificity of anti-HLA-DR1 alloreactive, human T cells was investigated by using DR1-expressing human and murine stimulator cells. All three bulk cell lines and six out of seven T cell clones proliferated in response to DR1-expressing mouse L cells. In addition to these species non specific T cells, three clones were identified which proliferated only in response to DR1 expressed by human or by murine stimulator cells. The patterns of response of these clones may reflect specificity for species or lineage-specific peptides with DR1. The results of aldehyde fixation and cytotoxicity experiments suggested that some of the T cell clones which proliferated in response to human and murine DR1 stimulators also required to recognize species-specific antigens. The responses of four of the six clones were abolished by fixation of DR1-L cells but not of a DR-1 EBV transformed lymphoblastoid cell line before co-culture. In addition, these clones were also cytotoxic for DR1-expressing human targets. The same clones which failed to recognize fixed L cells also failed to lyse DR1-L cells in a short term chromium release assay. Taken together these results suggest that some alloreactive anti-DR1, T cells are specific for peptides of cellular proteins seen in the context of the allo-MHC molecule. It is envisaged that L cells when co-cultured with human T cells, process and present peptides derived from proteins that are shed or secreted by the human cells, for co-recognition with DR1 on the L cell surface. The presentation of multiple peptides derived from endogenous proteins by allogeneic cells may contribute to the high precursor frequency of allo-reactive T cells.