We identified a noncovalent trimer of Sirurus asotus roe lectin (SAL) at Mr 95,362 along with its monomer at M-r 31,750 by electrospray ionization mass spectrometry when SAL was dissolved in 0.5% acetic acid, sprayed into the ion source with methanol as a sheath Liquid, and desolvated at 75 degrees C in a heated capillary column. The molecular weight of SAL, determined by the sedimentation equilibrium method, was 95,200 and the sedimentation coefficient (S-20,S-w) Of SAL in water was 5.58. SAL existed as a noncovalent trimer in solution and showed the ability to agglutinate rabbit erythrocytes. SAL showed three peaks (sal 1, sal 2, and sal 3) by C-8 reverse-phase HPLC, and these appeared to be a monomer, a dimer, and a trimer, respectively, by matrix-assisted laser desorption ionization-tine of flight mass spectrometry. sal 1 and sal 2 were shown to have a structure interchangeable with that of sal 3 in water. (C) 1997 Academic Press.