In this paper we show that eight of 17 tumor cell lines of various tissue origin constitutively express tumor necrosis factor (TNF) mRNA. Five of these eight cell lines concomitantly contained lymphotoxin (LT) mRNA. Of the remaining nine cell lines that lacked detectable TNF or LT gene expression, five could be induced by phorbol ester and/or cytokines to accumulate the respective mRNAs. While TNF mRNA was found not only in neoplastic hematopoietic cells, but also in cell lines derived from carcinomas, LT gene expression seemed to be restricted to lymphoid tumor cells. Tumor cells that expressed LT mRNA also secreted LT protein and proved to be resistant to the cytostatic/cytotoxic effects of their own protein product as well as to exogenous recombinant TNF and recombinant LT. In contrast, most of the cell lines containing TNF mRNA did not release TNF protein into the supernatants, indicating that TNF gene expression may be controlled predominantly at a post-transcriptional level. Thus, the presence of TNF mRNA does not necessarily reflect a TNF-resistant phenotype. Our findings demonstrate that TNF and/or LT mRNA expression is a rather common phenomenon in long-term cultured tumor cell lines and reveal that ectopic TNF and/or LT production by tumor cells may be involved in certain paraneoplastic syndromes as well as in tumorigenesis.