Streptokinase neutralisation titres were measured in a series of 25 patients who received streptokinase for the treatment of acute myocardial infarction. Before treatment, neutralisation titres were low (0.3 X 10(6) neutralisation units or less) in all patients. 3 months after treatment, neutralisation titres in 24 were such that a typical therapeutic dose of 1.5 million units of streptokinase would have been fully neutralised. At 4 1/2-8 1/2 months, 18 of 20 patients had neutralisation titres such that at least 50% of a dose of 1.5 million units of streptokinase would have been neutralised. After 8 months, neutralisation titres ranged from 0.4 to 2.0 million units in 8 patients. A decision to readminister streptokinase within 8 months (and probably up to 1 year) of previous thrombolytic treatment should take account of the neutralising capacity of plasma and the dose should be adjusted accordingly.