Serum level of cytokeratin 19 fragment; which is a tumor associated antigen of lung cancer; was detected by the radioimmunometric assay. Reliability for clinical laboratory measurement was investigated. Detective range of cytokeratin 19 fragment was 0-50 ng/ml and the results including reproducibility, linearity of the data from diluted standard samples, was good enough for laboratory practice use. Although, influence of high concentration of CA19-9 over cytokeratin 19 fragment assay was suspected, there was no correlation between amount of added the antigen to data of cytokeratin 19 fragment. So direct cross reactivity of cytokeratin 19 fragment antibody to CA19-9 seemed less possible. As inhibitory effect of samples which contained over 100 ng/ml of cytokeratin 19 fragment on assay was observed due to antigen excess, evaluation by diluted sample was necessary for the samples that showed over detective limit.