Serum samples from patients with Reiter's syndrome, ankylosing spondylitis, Yersinia-induced arthritis and Yersinia infections not followed by arthritis were collected and assayed for complement-dependent cytotoxicity, using a standard dye-exclusion microcytotoxicity technique. A total of eighty-nine serum samples derived from these patients were compared to those of forty control subjects, using as target cells six different cultured cell line cells and peripheral blood mononuclear cells isolated from the blood samples of four subjects. Serum samples from only five of the subjects demonstrated cytotoxicity. This did not depend on whether the target cells were HLA-B27 or not. Hence, if antilymphocytic auto-antibodies are present in these patients, they will require a different technique for their detection.