Phaseolus vulgaris leucoagglutinin is a homotetrameric legume lectin possessing the canonical dimeric structure common to other legume lectins. In order to gain insight into the stability of the protein in an acidic environment, it was characterized by CD and fluorescence studies at pH 2.5. This was then compared with the native protein at physiological pH (7.2). The extinction coefficient of the native protein was calculated to be 3.58x10(4) from its UV absorption spectra. The far- and near-UV CD spectra of the protein at pH 2.5 showed very little difference even though the protein was found to exist as a dimer at pH 2.5. The fluorescence emission maxima of the protein upon excitation at 280 nm were found to shift only from 331 nm at pH 7.2 to 333 nm at pH 2.5. Based on the above observation it was concluded that the protein exhibits extreme pH stability especially in the acidic range. The secondary and tertiary structure of the protein is lost only when it is incubated for two days in 6 M GdnHCl at pH 2.5. At pH 7.2 it could be denatured in 6 M GdnHCl after one week of incubation.