Antibodies to beta 2 microglobulin are found in systemic lupus erythematosus patients and are important in the lymphocytotoxic reactions of sera from such patients. In this study, beta 2 microglobulin antibodies were measured with the use of an enzyme-linked immunosorbent assay with purified beta 2 microglobulin antigen and peroxidase-labeled anti-human IgG or IgM. IgG antibodies to beta 2 microglobulin were found in 68% of 22 patients with ankylosing spondylitis. This incidence was higher than the 5% in 80 controls (P < 0.01) and similar to the 71% incidence found in 35 patients with systemic lupus erythematosus. Eleven (27%) of 41 patients with rheumatoid arthritis had elevated levels of antibodies to beta 2 microglobulin (P < 0.01). The mean antibody levels expressed in enzyme units were 0.125 for patients with ankylosing spondylitisis, 0.157 for those with systemic lupus erythematosus, 0.101 for those with rheumatoid arthritis, and 0.067 for controls. IgM anti-beta 2 microglobulin was not significantly different from controls. A competitive binding assay with enzyme-labeled beta 2 microglobulin was used to determine serum beta 2 microglobulin.