Serum beta 2-microglobulin (beta 2-M) measurements were carried out in 97 patients with monoclonal plasma cell disorders. Twenty-six (87%) of 30 patients with monoclonal gammopathy of undetermined significance (MGUS) had increased beta 2-M levels and serial follow-up in seven patients showed a progressive increase with time. Of the 63 patients with active myeloma, pretreatment serum beta 2-M values were available in 25 for correlation with pretreatment stage. Stage III beta 2-M levels were significantly higher than stages I and II (p less than 0.001). Four patients with smoldering myeloma had beta 2-M values similar to stage I disease. There was, therefore, excellent correlation between beta 2-M and myeloma tumor burden. Levels of beta 2-M decreased with response to chemotherapy induction and low levels in stable remission (plateau phase) were associated with unusually good prognosis. Median survival for stage III patients in stable remission with low serum beta 2-M was greater than 48 months. Conversely, at relapse very high beta 2-M levels were associated with a very fulminant and refractory course. Serum beta 2-M, therefore, appears to be an extremely useful marker in initial stratification and follow-up of myeloma patients.