A simple and inexpensive method is described for the determination of beta 2-microglobulin (beta 2-MG) by enzyme-amplified single radial immunodiffusion. The values obtained with this method correlate well with those determined by means of a commerical RIA kit. Using the immunodiffusion method we have measured the plasma levels of beta 2-MG in 135 patients with rheumatoid arthritis (RA) and normal serum creatinine levels. 33% of the patients had increased concentrations of beta 2 MG, but the levels were found to correlate poorly with the values of several variables generally used as indices of the degree inflammatory activity in RA. Furthermore, in contrast to earlier claims to the contrary, beta 2-MG correlated positively with age. The value of beta 2-MG in plasma as an index of inflammatory activity in RA is questioned.