An agglutination from seeds of 'alubia', a Mexican strain of Phaseolus coccineus, has been purified by affinity chromatography using physically entrapped stroma. The protein appears to be homogeneous by electrophoresis, molecular sieve chromatography and ultracentrifugation. A molecular species of approx. Mr 112,000, with S values of 6.25, 4.52, 4.63 and 4.65 at pH 2.5, 4.5, 7.0 and 9.5, respectively, consisting of four similar subunits (28 kDa), and containing 20% W/W glucosamine, is found to be responsible for the hemagglutinating capacity of 'alubia' extracts. No sugar able to inhibit agglutination has been found. The possibility that hemagglutination by Ph. coccineus var. 'alubia' involves cell receptors other than simple carbohydrate structures must therefore be considered.