A randomised double-blind placebo-controlled clinical trial was undertaken to assess survival after a single 30 unit intravenous dose of anisoylated plasminogen streptokinase activator complex (APSAC) within 6 h of onset of acute myocardial infarction. A planned interim analysis of mortality data was undertaken after 1004 patients had been followed for at least 30 days after randomisation. 61 (12.2%) of 502 patients on placebo died within 30 days compared with 32 (6.4%) of 502 patients on APSAC (p = 0.0016). Because of this 47% reduction in 30-day mortality (95% confidence interval 21-65%) patient entry to the trial was terminated. Preliminary 1 year mortality data show a similar trend. Percentage mortality reduction with APSAC was similar whether time since onset of symptoms was 0-4 h (660 patients) or 4-6 h (344 patients). Adverse events were few. These findings add to evidence that intravenous thrombolytic therapy after myocardial infarction offers a substantial reduction in mortality.