The effect of temperature on the time course of the precipitin reaction of concanavalin A with dextran was investigated by light scattering measurements at 350 nm in the metal-containing 0.01 M Tris-HCI buffer, pH7.4, and the metal-containing solution, pH 5.0. Analysis of the turbidity data according to the first-order kinetic law yielded the rate constant, k(obs). At pH 7.4 the curve between In k(obs) and 1/T was biphasic in the temperature range 8.6-37 degrees C, a sharp break occurred at 18 degrees C. The activation energy above 18 degrees C was 11.8 kcal/mol and below this temperature 37.0 kcal/mol. No temperature-induced aggregation of the dextran solution was indicated by the viscosity data in the desired temperature range (7-37 degrees C). The sharp break in the Arrhenius plot was attributed to the cold-induced dissociation of the concanavalin A tetramer into dimer.The absence of such a sharp break in the Arrhenius plot for the precipitin reaction at pH 5.0 where the lectin exists exclusively as a dimer supports this view. At pH 5.0 the activation energy was estimated to be 10.4 kcal/mol, which when corrected for the dependence of the rate constant upon the lectin concentration became 24.5 kcal/mol -- a value 2-fold higher than that found for the precipitin reaction at pH 7.4 and above 18 degrees C. The significance of the sharp break in the Arrhenius plot observed for the concanavalin A-dextran interaction in this study as well as for the lectin-mediated cell agglutination reported earlier has been discussed in terms of the involvement of multivalent interactions between complex ligands and the concanavalin A tetramer, which are not likely to be seen with simple sugars and the lectin with reduced valence.