The serum beta-2-microglobulin (B2-m) was evaluated by radio-immunoassay in 28 patients with ankylosing spondylarthritis. The mean B2-m level is within normal limits. There is no overall correlation with other biological or immunological parameters (Sed Rate, IgA, CD3+, CD4+, CD8+, Leu 7, NK activity). There is no difference between treated and untreated patients, between patients with recent or old disease. On the contrary, the mean the mean B2-m level is significantly lower in patients B27 positive, as compared with B27 negative. The possible role of anti-B2-m antibodies interfering with the serum B2-m assay discussed.