Binding of Concanavalin A to the Golgi complex and the resultant effect on intramembrane particles and enzyme activity has been studied with Golgi-rich fractions isolated from rat liver. Con A binds mainly to the tubular network of the Golgi complex and, to a lesser extent, the peripheral regions of the cisternae. Interaction of Con A with Golgi membranes is accompanied by aggregation of intramembrane particles reflecting increased membrane fluidity. Enhancement of galactosyl transferase, membrane bound in the Golgi complex, also occurs with Con A binding.