CYFRA is a new marker which measures a fragment of cytokeratin 19 in the serum by an immune radiometric method. The test is based on the preservation of the cytokeratin expression on the epithelial cells during the course of malignant transformation. In immuno-histochemistry the antibodies which selectively recognise cytokeratin react with all histological types of bronchial cancer. The presence of cytokeratin in the serum of patients suffering from cancer would be linked to their liberation during the course of cellular death. The threshold of specificity for CYFRA 21-1 is 3.3.3.6 ng/ml in a population suffering from benign respiratory diseases. The study performed in bronchial cancer produced the following conclusions: the marker is, above all, useful for epidermoid cancer; it is more discriminating than other markers to separate bronchial cancers and non-malignant respiratory disease. An elevated level of CYFRA 21-1 is predictive of advanced disease but does not permit any prediction as to inoperability. In 65% of cases, variations of CYFRA 21-1 are concordant with the stage of the disease during chemotherapy. Finally, elevated levels of CYFRA 21-1 predict a poor prognosis independent of the state of the disease.