Fatty acid synthase (FAS) is an enzyme which plays a central role in the de novo biosynthesis of fatty acids. FAS is selectively expressed in certain human cancers and therefore is a putative tumor marker. We developed an enzyme-linked immunosorbent assay (ELISA) for measuring FAS, and investigated its expression and clinical features. In this two-site sandwich ELISA, a polyclonal antibody was used as a capture on Nunc MaxiSorp ELISA/EIA modules and a monoclonal antibody labeled with biotin was used as a signal antibody. The assay was linear with no cross-reactivity with other tumor markers. The within- and between-run CVs were <10%, and the detection limit was 0.15 arbitrary Units/l. Recoveries were 92.4-105.1%. FAS was stable in buffer at 4 degrees C for more than 10 days and stable at 37 degrees C for 2 days. In human serum, FAS levels were significantly higher in patients with breast (1.01+/-0.71 Units/l, mean+/-S.D.), prostate (0.79+/-0.76 Units/l), colon (0.89+/-0.49 Units/l), and ovarian (0.84+/-0.9 Units/l) cancers compared to normal subjects (0.27+/-0.09 Units/l, P<0.01). This assay is sensitive, accurate, and precise and can distinguish between patients with various types of cancer and normal subjects.