Streptokinase (SK) is a bacterial protein used clinically as a thrombolytic agent in humans. Administration of SK causes a rapid increase in the frequency of anti-SK T cells and the titre of specific anti-SK antibodies that, on subsequent administration of SK, may neutralize the activity of the drug or elicit allergic-type reactions. By locating and modifying the immunogenic T-cell epitopes within the SK protein, it is possible that an agent with reduced immunogenicity but equal efficacy may be produced. We have investigated the T-cell epitopes within SK using nine non-overlapping, recombinant peptide fragments of SK. We investigated the proliferative T-cell response of peripheral blood mononuclear cells obtained from patients before and 6 days after administration of SK for myocardial infarction. We also examined the response of cultured anti-SK T-cell lines derived from patients 6 days after treatment with SK. Before administration of SK, peripheral blood mononuclear cells from six of nine patients showed a proliferative response to SK. The response was significantly higher 6 days after administration of SK ( P = 0.0004). Cultured T-cell lines showed similar proliferative responses to clinical-grade SK and recombinant SK. Marked differences in T-cell responses were apparent in response to each recombinant SK fragment ( P = 0.04). The mean proliferative response exceeded background to only two peptides, peptide 2 ( P = 0.04) and peptide 3 ( P = 0.009). Peptide 3, representing amino acids 100–150 of mature SK, was recognized preferentially in the majority of assays. Marked variation in the T-cell response to SK following treatment with this agent was observed between subjects. Despite these differences, peptides 2 and 3 induced T-cell proliferation at a level significantly above background in the majority of subjects. These epitopes may represent a region of enhanced immunogenicity within SK.