The present study was designed to clarify the mechanism by which some lung cancer cell lines can produce cytokeratin 19 (CK19) fragment and others cannot. We hypothesized that some lung cancer cell lines which cannot release CK19 express an incomplete sequence of CK19 mRNA. Expression of mRNA was evaluated by RT-PCR using several primer pairs for CK19. CK19 in the culture supernatant was measured by an immuno-radiometric assay. CK19 protein synthesis was evaluated by Western immunoblot and immunohistochemistry. Among 16 lung cancer cell lines, 7 released significant amounts of CK19 in the supernatant. In some cell lines, expression of CK19 mRNA was observed only in some combinations of primers, suggesting that incomplete mRNA was expressed. 3'-RACE analysis detected amplified products of a shorter size compared with normal amplified products in cell lines which expressed incomplete CK19 mRNA, suggesting that 3'-ends of mRNA for CK19 were deleted. Results of Western immunoblot and immuno-histochemical staining using anti-human CK19 monoclonal antibody completely correlated with the results on CK19 levels in culture supernatants as well as with complete expression of mRNA. We conclude that levels of CK19 closely relate to the expression of complete mRNA for CK19.