The in vitro transformation response of peripheral blood lymphocytes to human arterial and muscle homogenates has been studied in patients with polymyalgia rheumatica (PMR), giant cell arteritis (GCA) and in unrelated controls. Homogenates that stimulated PMR/GCA cells also stimulated controls. There was no greater stimulation to human or bovine elastin than to artery and muscle homogenates. Our results do not confirm a lymphocyte abnormality in these diseases.