A synaptic vesicle fraction and a synaptic plasma membrane fraction obtained after subfractionation of synaptosomes from chick forebrain have been used to produce antisera in rabbits. Immunofluorescence histology with the two antisera revealed that they reacted strongly with synaptic terminal regions present in the chick forebrain, cerebellum and spinal cord. In addition, the synaptic plasma membrane antiserum (but not the synaptic vesicle antiserum) reacted with preterminal axons in the cerebellum and spinal cord. Comparison of the two antisera by two-dimensional immunoelectrophoresis, revealed the presence of common antigens in the synaptosomal vesicle and plasma membrane fractions. Incubation of synaptosomes in vitro with the synaptosomal vesicle antiserum and complement produced a dose-dependent inhibition of synaptosome swelling up to a maximum of 55% of that obtained with the synaptosomal plasma membrane antiserum. The results of this test are consistent with the hypothesis that some synaptosomal vesicle antigens may be present also in the synaptosomal plasma membrane and imply that they face the external surface of the synaptosomes. The fate of vesicle membrane components in synaptosomal plasma membranes is not known. The possibility is discussed that they may be recycled locally by a mechanism similar to that proposed by Heuser and Reese (1973) for re-use of synaptic vesicle membranes at the neuromuscular junction.