Serum beta 2-microglobulin concentration was investigated in 221 patients with malignant lymphoma (Hodgkin type and non-Hodgkin type). In 55% of the unselected group of patients increased serum concentrations of beta 2-microglobulin were found. There was a close correlation between tumor extension and beta 2-microglobulin levels. In the tumor stage I the beta 2-microglobulin concentration was 1.65 +/- 0.45 mg/l, in the stage II 2.41 +/- 0.65 mg/l. The levels in the stage III (3.48 +/- 1.15 mg/l) and in the stage IV (5.49 +/- 1.99 mg/l) were significant higher than normal. Using longitudinal studies, measurement of beta 2-microglobulin was followed up during the course of the disease. During tumor regression by an effective antineoplastic therapy initially elevated beta 2-microglobulin concentrations decreased. In patients who achieved a complete remission a normalization of beta 2-microglobulin concentration was seen. We conclude from our findings that beta 2-microglobulin has the characteristics of a tumor associated marker in patients with malignant lymphomas. The low specificity and sensitivity limits the clinical use of this parameter.