The aim of the present study was to gain clinical experience with different dose levels of dalteparin, a low-molecular-weight heparin, following thrombolytic therapy in acute myocardial infarction. Compared to heparin, dalteparin has a longer half-life and a greater and highly predictable bioavailability, which would suggest dalteparin to be a convenient alternative. Twenty patients with ECG signs of acute transmural myocardial ischemia received streptokinase (1.5 million IU for 60 min) and were allocated to a control group or to open treatment with 50, 75 or 100 IU of dalteparin/kg b.w. s.c. b.i.d., starting 4 h later, for 6 days. Each group consisted of 5 patients. Except for the control group, aspirin was withheld during dalteparin treatment. Anti-factor-Xa (anti-FXa) values increased dose-dependently during the first 24 h and were maintained throughout the study period. On day 6, anti-FXa levels after 100 IU/kg b.w. were 0.79 (0.59-1.00) IU/ml (median, min.-max.) 4 h after administration of dalteparin, and 0.51 (0.34-0.82) IU/ml before the subsequent dose of dalteparin. In conclusion, our results indicate that a dalteparin dose slightly higher than 100 IU/kg b.w. is required in order to obtain the presumed therapeutic range of anti-FXa (0.6-1.0 IU/ml).