Synaptosomal plasma membranes (SPMs) from adult rat brain tested for fibronectin binding and antigenicity toward antibodies against integrin-type fibronectin receptors. Binding (1-10 pmol mg-1 protein) was considerably greater in SPMs than in homogenates for hippocampus and neocortex but not for brain stem. The tetrapeptide L-arginyl-glycyl-L-aspartyl-L-serine blocked up to 90% of the binding indicating that integrin-type receptors are present. Antibodies against two sub-classes of integrins detected high Mr integrin subunits in homogenates but not in SPMs. However, antibodies against hamster ovarian fibronectin receptor reacted intensely with polypeptides of 55,000 and 40,000 Mr that were concentrated 20 to 40-fold in SPMs compared to homogenates. These peptides may constitute a new class of matrix receptor involved in synaptic adhesion.