Using allogeneic antibody, we previously described a highmolecular-weight glycoprotein in the urine of 68% of melanoma patients. This glycoprotein has been termed urinarytumor-associated antigen (U-TAA). A murine monoclonal antibody (MAb) specific for U-TAA (AD1-40F4) has been developed. By the use of AD1-40F4, U-TAA was detected in serum samples from 63% (33/52) of stage II and stage III melanoma patients, but from only 5% (1/20) of normal controls. This report describes the physical and immunochemical properties of U-TAA in the serum. The antigen elutes from a DEAE-Sephacel column in association with IgG in the void volume and as free antigen in a second peak. The molecular mass of the free antigen is 590–620 kDa and it sediments in the region of 28–29% sucrose by density gradient ultracentrifugation. Free antigen has an isoelectric point of 6.1. This high molecular weight antigen is composed of smaller subunits linked by reducible bonds. The AD1-40F4 reactive epitope resides on a 90–100 kDa subunit. These results provide evidence that U-TAA which is produced by melanoma cells in vitro is present in the circulation of melanoma patients.