The purpose of this study was to investigate whether, to what extent, and through which mechanisms intravenous heparin, administered before and after streptokinase, affects the plasma levels of D-dimer and fibrinogen in myocardial infarction. Data concerning mortality and incidence of coronary recanalization in patients receiving heparin and thrombolytic therapy after acute myocardial infarction are controversial; furthermore, the mechanisms through which heparin acts in combination with thrombolytic therapy are unclear. Thirty-eight patients with acute myocardial infarction treated with streptokinase were considered. Nineteen of them received, immediately before the beginning of thrombolytic treatment, a bolus of heparin (100 U.kg-1 intravenously) and, 2 h later, intravenous heparin in doses raising the partial thromboplastin time to 2-2.5 times the normal value (Group 1); the remaining 19 did not receive anticoagulant treatment (Group 2). Multiple determinations of plasma D-dimer and fibrinogen levels were obtained in all patients before, and in the seven days following thrombolytic treatment. Six hours after streptokinase, fibrinogen decreased from 304 +/- 34 to 61 +/- 34 mg.dl-1 in Group 1 and from 312 +/- 29 to 38 +/- 21 mg.dl-1 in Group 2 (P < 0.02 versus Group 1). The same difference between groups persisted at the 12th and at the 18th hour. D-dimer values, from 0.5 +/- 0.1 microgram.dl-1 in Group 1 and 0.4 +/- 0.1 microgram.dl-1 in Group 2, increased at the 1st hour to 37.2 +/- 36.5 micrograms.dl-1 and 52.2 +/- 39.8 micrograms.dl-1, respectively. A peak value was reached in both groups at the 6th hour, which was followed by a slow decrease.(ABSTRACT TRUNCATED AT 250 WORDS)