Serum beta 2-microglobulin (S-beta 2m) was measured at diagnosis in 189 patients with malignant lymphoma, all with a normal serum creatinine clearance. The diagnosis was non-Hodgkin's lymphoma (NHL) in 149 patients and Hodgkin's disease (HD) in 40. Among the NHL group, S-beta 2m was raised (greater than 3.0 mg/l) in 15% of patients with Stage I and II and in 65% of those with Stage III and IV. The corresponding frequencies for HD were 11% and 83% respectively. In NHL, a high pretreatment level of S-beta 2m was found to be a poor prognostic sign in all stages. Patients in Stage I and II with an elevated pretreatment level of S-beta 2m had a higher relapse rate than those with normal S-beta 2m. In Stage III and IV patients with initial levels greater than 3.5 mg/l, the survival was significantly shorter than in those with initial values of less than 3.5 mg/l. A lower mean S-beta 2m value was found in Stage III and IV patients who achieved complete remission than in those who did not. Serial determinations of S-beta 2m in 23 patients with NHL showed that increased pretreatment levels became normal when remission was achieved and increased again in relapse. Thus the S-beta 2m provides valuable prognostic information in this group of patients.