In 319 patients who participated in a double-blind trial to evaluate the effect of early rt-PA administration compared to placebo in suspected acute myocardial infarction, infarct size was assessed from analyses of serum activity of lactate dehydrogenase isoenzyme 1 (LD 1). Treatment was always started less than 3 h after the onset of symptoms, with one-third of the patients' treatment being initiated outside the hospital. The maximum activity of LD 1 was reduced by 32%, from 13.3 mu kat l-1 in placebo to 9.0 mu kat l-1 in rt-PA treated patients (P = 0.001). A reduction in LD-1 activity after rt-PA treatment was restricted to patients with ST-elevation in the initial electrocardiogram, and was more pronounced in patients with previous ischaemic heart disease, above median age, and in those with a shorter delay in initiation of treatment. We conclude that very early intravenous treatment with rt-PA limits indirect signs of infarct size. The effect appears to be restricted to patients with ST-segment elevation in their initial electrocardiogram.