An immortalized fibroblast cell line, designated as CCFS-1/KMC, derived from human neonatal foreskin fibroblasts, contained human papillomavirus (HPV) type 18 DNA. Since this newly established cell line could spontaneously secrete activating factors for normal human blood lymphocytes, the synthesis and release of potent inflammatory cytokines from this cell line were checked. To determine the presence of cytokines in the supernatant collected from the cell line, tests by a cytokine-specific ELISA and a mitogenesis bioassay were done. The cell line could spontaneously produce several immunoreactive cytokines, such as tumor necrosis factor (TNF), interleukin 1 (IL-1) and interleukin 6 (IL-6). It also could stimulate the mitosis of human blood lymphocytes and Raji lymphoblast cell line. These cytokines were present in the same fraction of isoelectric points (pI) from 5.4 to 5.6. This study suggests that non-immune bystander cells may exert immunomodulatory effect on the immune cells via the production of potent inflammatory cytokines during viral infection.