Vegetative (noncohesive) D. discoideum cells and cells at several stages during the development of cohesiveness were fixed with glutaraldehyde, and their agglutinability by purified carbohydrate-binding proteins (lectins) from slime molds and plants was determined. The two purified lectins from D. discoideum, called discoidin I and II, were poor agglutinins of fixed vegetative D. discoideum cells and potent agglutinins of fixed cohesive D. discoideum cells. The increased agglutinability of D. discoideum cells by discoidin I or discoidin II with differentiation paralleled discoidin appearance in cell extracts and development of cohesiveness. The lectin of another slime mold, Polysphondylium pallidum (pallidin) and the lectin of Ricinus communis (RCA-I) were also more potent agglutinins of fixed differentiated D. discoideum cells than of fixed vegetative D. discoideum cells. However, wheat germ agglutinin was much more potent with fixed vegetative cells. The association constant (Ka) of discoidin I and discoidin II for fixed differentiated D. discoideum cells was in the range of 10(9) M-1, more than twenty-fold higher than the Ka of these lectins for fixed vegetative cells. The Ka of discoidin I and discoidin II for fixed differentiated P. pallidum cells was an order of magnitude lower than for fixed differentiated D. discoideum cells. The Ka of pallidin for fixed differentiated P. pallidum cells was about 4 X 10(9) M-1 and was an order of magnitude lower for fixed differentiated D. discoideum cells. Since previous studies showed that the lectin of each of these slime mold species is detectable on the surface of cohesive cells, the observation of species-specific receptors on the surface of cohesive cells suggests a basis for species-specific cohesion.