We investigated the clinical significance of the serum soluble interleukin-6 receptor (sIL-6R) in 42 patients with plasma cell dyscrasias (27 with multiple myeloma (MM), 13 with monoclonal gammopathy of undetermined significance (MGUS), and two with plasma cell leukaemia (PCL)). Serum levels of sIL-6R in normal individuals were 77 +/- 21 ng/ml (mean +/- SD, n = 18); those in patients with MGUS and with MM were elevated (102 +/- 33 ng/ml, mean +/- SD, P < 0.05 and 126 +/- 60 ng/ml, mean +/- SD, P < 0.01, respectively). Significant correlations were not found between the serum levels of sIL-6R and known prognostic factors (C-reactive protein, haemoglobin levels, calcium, creatinine, beta 2-microglobulin, amounts of M-protein, or percentages of plasma cells in bone marrow). Elevated serum sIL-6R did not affect the survival of the patients with MM. Serial measurements of sIL-6R together with the clinical course of patients with plasma cell neoplasias revealed a good correlation between the sIL-6R level and disease activity. We conclude that sIL-6R can be used as a clinical factor correlated with the disease activity, at least in some patients with plasma cell neoplasias.