A cell line designated MES-SA has been developed from a uterine sarcoma. Cells from the surgical tumor specimen were grown in a soft-agar clonogenic assay, with a relatively high plating efficiency of 0.5% and sensitivity to multiple drugs. Histologically, the surgical specimen and tumors developing after MES-SA inoculation into nude mice were identical, consisting of sheets of anaplastic sarcoma cells amid scant hyalinized stroma. The nonepithelial origin of this line was supported by ultrastructural analysis and negative mucin staining. Growth in monolayer was established by seeding colonies from soft agar into liquid media and has been maintained for over 21 months (greater than 100 passages), with a population-doubling time for the cell line of 22 hr. The MES-SA line readily forms colonies in soft agar with plating efficiencies ranging from 10 to 20%. Tumor cell inoculation s.c. into nude mice produces tumors within 2 to 3 weeks and subsequent tumor volume-doubling times of 7 to 10 days. MES-SA has a modal chromosome number of 45. Karyotypic abnormalities include: monosomic forms of chromosomes 5, 6, and 7; a 5q, 6p translocation; and one marker chromosome. In vitro sensitivities to doxorubicin, dactinomycin, mitomycin C, and bleomycin have been demonstrated by clonogenic assay. These drug sensitivities remain stable over long periods of monolayer growth and after passage in nude mice.