BACKGROUND Thrombolysis with recombinant tissue-type plasminogen activator (rt-PA) is successfully used in acute myocardial infarction with ST elevation (STEMI). Reocclusions follow rt-PA treatment in up to 30% of patients within one year. The infusion of rt-PA may induce the production of anti-t-PA antibodies which could interfere with the function of the native t-PA molecule. METHODS In order to detect and characterise anti-t-PA antibodies, plasma samples were collected from 30 STEMI patients (20 treated and 10 not treated with rt-PA) at baseline before rt-PA infusion and then 15, 30, 90 and 180 days after STEMI and from 40 healthy subjects at baseline only. Immunoenzymatic, chromatographic and chromogenic methods were employed. RESULTS An increase of anti-t-PA antibodies was observed 15 days (IgM, p=0.0001) and 30 days (IgG, p=0.0001) after rt-PA infusion. Six patients had large increases of anti-t-PA IgG which bound the catalytic domain of t-PA (two cases) or kringle 2 domain (four cases), were of IgG1 or IgG3 subclasses and interacted with the t-PA molecule in fluid phase. CONCLUSIONS The infusion of rt-PA may induce the production of specific antibodies that bind active sites of t-PA, thus potentially reducing its in vivo function.