A new human tumour cell line (designated HX 165c) has been established from an epithelioid sarcoma presenting in a 28 year old male. The cells grew as an adherent monolayer with a doubling time of 38 h and had mainly epithelial morphology but with areas of mesenchymal-like cytoplasmic extensions. The mixed epithelial-mesenchymal phenotype was also apparent by intermediate filament analysis which showed reactivity to vimentin and keratin. The cells were tumorigenic in nude mice and aneuploid, possessing a mean chromosome number of 65. In vitro cloning determinations gave colony-forming efficiencies of 0.01% in an anchorage-independent soft agar assay and 34% in a monolayer anchorage-dependent assay. The cells were in the mid-range for radiosensitivity of human tumour cells (surviving fraction at 2 Gy of 0.39). In addition, experiments utilising continuous low dose rate irradiation at 3.2 cGy min-1, showed that the cells possessed only a small capacity to recover from radiation damage (dose reduction factor at 1% cell survival of 1.15 for 150 versus 3.2 cGy min-1). This cell line, being only the second we are aware of to be established from this rare soft tissue sarcoma, should be useful in helping to ascertain the histogenesis of epithelioid sarcoma.