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 was 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 mRNA produced by neoplastic B cells and serum 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.