A prospective study of 62 patients with systemic lupus erythematosus (SLE) was performed in order to establish whether serum beta 2m could be a good marker of clinical activity. beta 2m was determined by radio-immunoassay and the values compared with a control group of healthy individuals. The mean value of beta 2m in the control group was 1.48 +/- 0.52 mg/l and 2.87 +/- 2.19 mg/l (p less than 0.001) in the SLE group, 4.53 +/- 2.89 mg/l in the 22 patients with active disease and 2.40 +/- 1.80 mg/l (p less than 0.001) in the 40 patients with inactive disease. High beta 2m values (greater than or equal to 3 mg/l) were observed in 64% of the patients with active SLE, VS. only 12% of the patients with inactive disease (p less than 0.001). Significant differences were also observed when beta 2m of the patients with inactive SLE was compared with the control group (p less than 0.001). The beta 2m with a sensitivity of 64%, a specificity of 87%, and an efficiency of 79% could be a good parameter to detect SLE clinical activity.