We have defined the minimum length of a synthetic peptide which can activate I-Ed-restricted BALB/c T cell clones specific for a mutated self-antigen: an idiotope on the syngeneic lambda 2315 immunoglobulin light chain. A peptide comprising residues 91-101 of the lambda 2315 sequence had full stimulatory potency. Surprisingly, a peptide analogue in which His97 was deleted was almost fully active. Truncated, deleted or substituted peptide analogues did not distinguish between seven T cell clones that use different alpha/beta T cell receptors. The 91-101 region in the lambda 2315 light chain does not form an amphipathic helix even though such a helix has been suggested to be important for T cell epitopes. Further, a motif proposed by Rothbard and Taylor as being common to T cell immunogenic peptides is not necessary for the lambda 2315 idiotypic peptide. Comparison with seven other I-Ed-restricted peptides revealed that the peptides are generally positively charged and have two basic amino acids clustered around the centre. On the basis of a model of the class II molecule peptide binding site, we suggest that these positively charged residues may interact with the negatively charged residues at positions 114(Glu) and 155(Asp) of the E beta d chain.