Hydrogen ion titration of an affinity-purified mannose/glucose-specific lectin from Cajanus cajan pulse was carried out at 30°C and ionic strength of 0.15 by a discontinuous method. The titration was reversible in the pH range 2–12.0. The numbers of different ionizable groups per 39,000 g of the lectin were 43 carboxyl groups (pKint = 3.93), 10 imidazole groups, 21 ε-amino groups, 12.8 phenoxyl groups (pKint = 10.0), and 5 guanidyl groups. Only seven tyrosine residues of the lectin were dissociated under native conditions. The remaining six tyrosines became available for titration upon denaturation of the lectin in 9 M urea.