This antiserum against Human Thymidylate Synthase (hTS) has been tested for use in ELISA, immunoprecipitation, immunofluorescence microscopy, immunohistochemistry and immunoblotting. The antibody recognizes the expected additional band corresponding to the ternary complex of hTS-dFUMP-reduced folate in HeLa cells treated with the TS inhibitor 5-FUdR. This event occurs in most human breast, colorectal, gastric, head and neck carcinomas. The antibody recognizes the 36,000 MW hTS. Reactivity in other immunoassays is unknown.