Cells of the proerythromyeloid cell line K562 and of the T cell line Jurkat were able to induce an increase in TNF-, IL1 alpha-, and IL1 beta-mRNA expression in human peripheral blood derived monocytes in vitro. The activating principle of these tumor cells was associated with fractions of membrane preparations of distinct molecular mass, 32-38 kD for Jurkat cells and 46-54 kD for K562 cells, respectively. At least part of the activating constituent seemed to be protein in nature. Isolated membrane preparations of both cell types induced production and secretion of TNF. Stimulation of monocytes with the viable tumor cells led to TNF release only when Jurkat cells were used. Viable K562 cells induced enhanced TNFmRNA expression but seemed to absorb soluble TNF from the supernatant.