The nature and density of coronary arterial inflammatory infiltrates were studied in autopsy material from 53 patients with no clinical or pathological evidence of ischaemic heart disease and from 22 acute cardiac deaths. Adventitial aggregates of inflammatory cells were present in 46% of cases and were slightly more frequent in arteries from patients with ischaemic heart disease. The majority of these cells were lymphocytes and immunohistochemical studies with monoclonal antibodies indicated that both B and T cells were present. Staining with polyclonal antibodies to macrophage antigens confirmed that some adventitial cells were macrophages. The reactions of foam cells within the atheromatous plaques were extremely variable but some foam cells gave unequivocably positive reactions with macrophage antibodies. There was a good overall correlation between the numbers of intimal and medial foam cells and the density of the adventitial lymphocytic infiltrates. It is suggested that these infiltrates develop as a secondary feature of atheromatous lesions and are unlikely to play a major role in their initiation or in the development of complications.