A neutrophil chemotactic factor (human interleukin 8, human granulocyte-macrophage colony-stimulating factor)-producing cell line, named KHM-5M, was established from a patient with an undifferentiated thyroid carcinoma, neutrophilia, and malignant pleurisy with many neutrophils and a few malignant cells. The cell line was transplanted into nude rats, and the infiltration of neutrophils was observed in and around the transplanted tumor tissue. Neutrophil chemotactic activity was predicted from the clinical features and pathological findings in this case. The extreme chemotactic activity of the neutrophils was demonstrated in conditioned medium from KHM-5M cells using the modified Boyden chamber technique. With sodium dodecyl sulfate-polyacrylamide gel electrophoresis, at least two neutrophil chemotactic activities in conditioned medium from the cell line were observed. The levels of these activities derived from KHM-5M cells were screened by measuring conditioned medium from the COS cells, which expressed a complementary DNA library from the KHM-5M cells. Chemotactic activities (human interleukin 8, human granulocyte-macrophage colony-stimulating factor) were identified by DNA cloning. These results show that the KHM-5M cells derived from an undifferentiated thyroid carcinoma produce multicytokines and suggest that those cytokines modified some pathological features in this case.