Pretreatment serum beta 2m levels were estimated in 65 patients with multiple myeloma. After correction for changes in renal function, the level of beta 2m attributable to multiple myeloma (beta 2m-m) no longer correlated with the serum creatinine, but showed a strong correlation with the total body myeloma cell mass (P = 0.002). There was an inverse correlation between beta 2m-m and the Karnofsky performance status of patients at presentation (P = 0.001). Patients with a pretreatment beta 2m-m level of less than the median value of 2.94 mg/l survived significantly longer than those with a raised level (P = 0.0008).