Amino-acid compositional and sequence analyses as well as mass spectrometric determinations of purified rat urine proteins, previously termed prealbumin and alpha(2)-euglobulin, have revealed a high homology between the two forms which have now been identified as alpha(2)-globulin species. The "prealbumin' fraction was found to correspond to alpha(2u)-globulin originating from salivary gland and the 'alpha(2)-euglobulin' fraction was identical with the major urinary protein (MUP) or alpha(2u)-globulin. The results indicate that the two major protein fractions of rat urine constitute different forms of the same parent protein, alpha(2u)-globulin, having no amino-acid sequence resemblance to prealbumin (transthyretin) of rat serum.