In the prion diseases, a prolonged, asymptomatic incubation period precedes the onset of neurologic dysfunction. At present, a noninvasive test is not available for the presymptomatic diagnosis of prion disease, and thus the report of a test for prions using urine has been of great interest (Shaked, G. M., Shaked, Y., Kariv-Inbal, Z., Halimi, M., Avraham, I., and Gabizon, R. (2001) J. Biol. Chem. 276, 31479-31482). Using Western immunoblots with the anti-prion protein (PrP) 3F4 monoclonal antibody and an anti-mouse IgG secondary antibody, a protease-resistant PrP was reported in the urine of Syrian hamsters and humans with prion disease. Here we have demonstrated that this purportedly "protease-resistant PrP" band in the urine of diseased hamsters is detectable using the anti-mouse IgG secondary antibody in the absence of the 3F4 monoclonal antibody. Mass spectrometric analysis identified an immunoglobulin light chain in the band but found no PrP peptides. No similar band was found in the urine of uninfected hamsters or in brain homogenates from normal or prion-infected hamsters. Moreover, the band in the urine of infected hamsters was not detected using two chimeric human-mouse recombinant anti-PrP antibody fragments followed by an anti-human IgG secondary antibody. Our results indicate that the band detected under previously published conditions is due to the cross-reactivity of the anti-mouse IgG antibody with IgG light chains and possibly heavy chain fragments in urine, but not with PrP.