To determine the distribution of antibodies to streptokinase that might be anticipated in patients requiring treatment with streptokinase, specific anti-streptokinase antibody titres were determined in a group of subjects from the general population and in a group of patients presenting with acute myocardial infarction. Enzyme-linked immunosorbent assays were developed to measure specific anti-streptokinase IgG and subclass IgG1 in 95 subjects from the general population and in 160 patients presenting with acute myocardial infarction. Low titres of IgG1 were found in both the general population (median = 5; range: 0-490) and in the myocardial infarction group (median = 7; range: 0-2000). A minority of subjects in both groups had high titres. The findings suggest that low titres of antibody are widespread in the population. The minority of subjects in both groups who had high titres may explain the infrequent type III immune reactions encountered with streptokinase.