Circulating immune complexes were found in 2 of 15 patients with giant cell arteritis (GCA) by using a solid phase Clq enzyme linked immunoabsorbent assay (ELISA). The prevalence in controls was 5%. Rheumatoid factor could be demonstrated in 2 out of 27 patients and in 11% of the controls by using a similar ELISA technique. The prevalence of T cells in blood was similar in 25 patients with GCA and in controls. The blood lymphocyte blastogenic response to the mitogens, phytohaemagglutinin, concanavalin-A, and pokeweed mitogen did not differ in 25 untreated patients compared with controls. Stimulation of lymphocytes by arterial homogenates was tested in 8 patients. In no case could a significant simulation by obtained. We conclude that immune complexes and rheumatoid factors are present in the same low frequency in GCA patients as in the normal population, and that the studied parameters of cellular immunity appear to be normal.