Superprotein is a rapidly axonally transported protein that is conspicuously labeled with 35S-methionine supplied to the cell bodies of retinal ganglion cells. Superprotein candidates are apparent among the rapidly transported proteins of many neurons from the CNS and PNS, including cranial, sympathetic, sensory, and motor neurons from mammals, fish, and amphibians. To determine the identity of Superprotein, we purified it from rabbit visual system and spinal cord and determined the amino acid sequence of seven of its tryptic peptides. The sequence shows that Superprotein is SNAP-25, a protein recently predicted from a cDNA sequence; SNAP-25 has been reported to be concentrated in the synaptic terminals of a selected population of CNS neurons. We measured the amount of radioactivity associated with Superprotein in tissue containing axons (optic tract) and synaptic terminals (superior collicules) of rabbit retinal ganglion cells. Labeled Superprotein disappeared from the superior colliculus more rapidly than another protein (synapsin I-like protein) that is concentrated in synaptic terminals. These results serve to unite the observations on the synthesis, distribution, metabolism, and axonal transport of Superprotein with observations of SNAP-25 and its mRNA.