A cell line was established from a nodule of human giant and spindle cell carcinoma of the thyroid metastatic to a kidney. The culture was maintained through more than 24 passages over a period of 1 1/2 years. Its identity as a cancer cell line was established by its growth on heterotransplanta-tion into immunologically tolerant rats with reproduction of the histologic appearance of the original tumor. Chromosomal studies showed a high degree of aneuploidy and polyploidy with numerous abnormalities of chromosomal form.