Abstract Under the same conditions generally used for agglutination tests, the concanavalin A molecule undergoes quarternary structure transitions at different temperatures. The tetrameric form is predominant at 37 °C but it dissociates into dimers as the temperature is lowered. Although the dimer does not induce agglutination at low temperature, electron microscopic studies show that it can bind free exogenous glycoproteins. It is suggested that the effects of temperature on surface membrane receptor sites and on agglutination, at least with this lectin, cannot be interpreted exclusively in terms of cell surface properties. The dimer-tetramer transition of concanavalin A and the possible resulting steric effects have to be taken into account when evaluating the receptor sites number from binding experiments.