AbstractA new human skin cell line, designated as CCFS‐1/KMC, immortalized from human neonatal foreskin diploid fibroblast cells, has been subcultured successfullyin vitrofor more than 500 passages. This anchorage‐dependent cell line possesses many common features of transformation such as morphological and cytoskeletal changes, hypotriploidy, infinite lifespan, increasing plating efficiency and saturation density, and decreasing serum requirement and population doubling time. Human papillomavirus (HPV) type 18 DNA was detected in the cell line before and after immortalization by the polymerase chain reaction (PCR) method. Tumorigenicity, however, was not demonstratedin vivo. The isoenzyme activity of the cell line shows activation of a placental form of alkaline phosphatase and a changing lactate dehydrogenase isoenzyme pattern that is different from transformation by carcinogens. Class I HLA and class II HLA antigens are constitutively expressed on this skin cell line. Here we report that these immortalized human fibroblasts derived from neonatal HPV‐18‐DNA‐contained diploid fibroblasts possess double minute chromosomes (DMs), a karyotypic aberration usually found in can