The in vitro proliferative response to purified allergens of the grass, Lolium perenne pollens was studied using PBMC from individuals allergic to grass pollens and Ag-specific T cell lines and T cell clones derived from them. The PBMC from all 10 subjects studied showed a strong response to Lol p I and most of them (8 of 10) also responded to Lol p III. Although Lol p II induced a moderate response in 4 of 10 individuals, it did not induce any response in others at all the Ag concentrations tested. However, one of the subjects (JH) responded to, besides Lol p I, both Lol p II and Lol p III equally well. Analysis of Ag-specific T cell lines and clones derived from three individuals showed varied pattern of reactivity to the Lol p allergens. Some of the Lol p III-specific T cell lines and clones were also stimulated by Lol p I and similarly, some of the Lol p I-specific T cell clones (derived from four other subjects) were stimulated by Lol p III; thus showing a two-way cross-reactivity between those T cells. In both cases, the cross-reactivity to Lol p II, when observed, was lower than that seen with Lol p I and Lol p III. Comparison of amino acid sequences of the three Lol p proteins revealed a significant level of structural similarity among them, including several segments of identical sequences. Although one of the synthetic peptides of Lol p III sharing appreciable sequence homology with other proteins stimulated PBMC from two subjects, three other peptides did not. Nevertheless, these studies indicated the possible existence of cross-reactive T cell epitope(s) among the grass pollen allergens. Based on these results, the relationship between amino acid sequence homology among the Lol p proteins and their recognition by T cells is discussed.