The presence of soluble tumor necrosis factor (TNF) binding proteins (BP) was investigated in the sera of healthy volunteer blood donors and cancer patients. Two distinct types of TNFBP, types A and B, which are immunologically related to the cellular 75-kD TNF receptor (TNFR) and the cellular 55-kD TNFR, respectively, were assessed by immunoassays using nonblocking anti-receptor antibodies and 125I-recombinant human TNF alpha. As compared to the titers observed in 25 healthy controls, TNFBP types A and B titers were found to be elevated in almost all sera obtained from patients with underlying malignant disease. The highest amounts of TNFBP were seen in the sera of patients with B cell malignancies including hairy cell leukemia (HCL) and type B chronic lymphocytic leukemia. Treatment of HCL patients with recombinant human interferon-alpha was associated with decrease of circulating TNFBP.