Five cell lines were established from four undifferentiated carcinomas and one squamous cell carcinoma of the thyroid. The levels of several kinds of cytokines were measured in the conditioned media of these cell lines by enzyme-linked immunosorbent assay (ELISA). Interleukin-6 (IL-6) was produced by four of the five cell lines, interleukin-1 alpha (IL-1 alpha) by three cell lines, and granulocyte-colony stimulating factor (G-CSF) by two cell lines. The mRNA of IL-1 alpha or IL-6 was detected by Northern blot analysis in all the cell lines which secreted these cytokines into culture medium. These results suggest that undifferentiated carcinoma and squamous cell carcinoma of the thyroid frequently produce cytokines. Further studies are needed to clarify the possible clinical effects of these cytokines in patients with thyroid carcinoma.