Cases of ischaemic heart disease have been studied for the cell-mediated immune response against human heart antigen by using the leucocyte migration inhibition test. In 30 cases of acute myocardial infarction, the leucocyte migration inhibition values started increasing from the first week reaching a peak in 3 to 4 weeks and then declining but still above control values 12 months after infarction. The leucocyte migration inhibition values were significantly higher than control values in another 10 patients with late complications of previous infarction and in those patients with acute myocardial infarction who were less than 40 years of age, who had extensive anterior infarction, or who had a past history of angina pectoris. The leucocyte migration inhibition values were negligible in all the 12 patients with stable angina pectoris, but were high in 2 of the 8 with unstable angina and in 3 of the 4 with the intermediate coronary syndrome. The leucocyte migration inhibition values were much higher in patients with complications, which may be the result of cardiac damage by a cell-mediated immune response.