We investigated the expression of interleukin-6 (IL-6) mRNA in tumours from 21 patients with B-cell non-Hodgkin's lymphoma (NHL). Total RNA, extracted from diagnostic tissue specimens, was subjected to semi-quantitative analysis for IL-6 mRNA by the reverse transcription-polymerase chain reaction (RT-PCR). The amount of IL-6 mRNA ws semi-quantified against that in pokeweed mitogen (PWM) stimulated peripheral blood mononuclear cells. The expression of IL-6 in neoplastic B cells was confirmed by immunohistochemistry. We then looked for correlations between the amount of IL-6 concentrations. The amount of IL-6 mRNA correlated with the circulating IL-6 concentration, suggesting that the malignant cells were the source of IL-6 in these patients with B-cell NHL. Our results suggest that the circulating IL-6 in patients with B-cell NHL is derived from the neoplastic lymphoid cells, and that neoplastic cells may act as modulators of the general status of patients with B-cell NHL.