Lectins are toxic heat-stable glycoproteins, termed phytohemagglutinins, that depress the nutritional quality of Phaseolus vulgaris. Purified phytohemagglutinin (PHA-P) was separated from dark red kidney beans (Montcalm cultivar) by affinity chromatography and subjected to various treatments including: thermal (70–100C; chemical (2 M NaCl, 5 M urea, 5% mercaptoethanol, pH 12 and pH 3) and enzymatic (peptidases, proteases and carbohydrases). At 70C, the PHA-P activity decreased with a linear response. Exposure to pH 12 or 5 M urea were the most effective chemical treatments for reducing hemagglutinating activity. Treatment with proteases resulted in 88–98 % reductions in PHA-P activity. Man-nosidase treatment resulted in a slight but significant (P < 0.01) reduction in activity; however, amylases and neuraminidase had no effect on activity. Terminal sialic acid residues may not be an important determinant of PHA-P activity.