Mycoplasma fermentans-derived high-molecular-weight material (MDHM) was originally described to induce differentiation of murine thymocytes to cytolytic effector T-cells by stimulating IL-6 release from adherent cells. This study shows that human peripheral blood monocytes (PBMo) also respond to MDHM with increases in IL-1 beta, IL-6 and TNF alpha expression, both at the mRNA and protein level. The induced expression of IL-1 beta and TNF alpha mRNA in the monocytic THP-1 cell line increased as quickly as in primary cells. In contrast to PBMo, THP-1 and 14 other monocytic/myeloid leukemia-derived cell lines did not secrete measurable amounts of the cytokines upon treatment with MDHM. IL-1 beta and IL-6 genes contain AP-1 binding sites as regulatory elements, the AP-1 protein being composed of c-jun and c-fos gene products. In THP-1 cells c-jun mRNA expression increased after incubation with MDHM while positive c-fos expression remained unaffected. Although these data suggest AP-1 regulated cytokine mRNA expression, results from PBMo are not in accordance with this notion. In the primary cells MDHM-induced elevation of cytokine mRNA levels was preceded by a downregulation of c-fos expression while positive c-jun expression was not modulated. c-myc mRNA expression, constitutively high in THP-1 cells, was induced in MDHM-stimulated PBMo. In conclusion, MDHM-stimulated induction of cytokine mRNA expression was accompanied by different proto-oncogene responses in PBMo and THP-1 cells. These differences may represent different regulatory pathways of the two cell systems. Alternatively, these data support the notion that neither AP-1 nor the c-myc protein are involved in the MDHM-induced increase in IL-1 beta, IL-6 or TNF alpha mRNA levels. Furthermore, the present results demonstrate clearly that mycoplasma products can have a profound impact on the activation status of eukaryotic cells.