Two vesicular fractions and one nonvesicular fraction were prepared from crude synaptosomes by differential centrifugation and salting out with ammonium sulfate. Fraction 1 contained a mixture of coated vesicles, material thought to be derived from breakdown of the coats (shell fragments), and plain synaptic vesicles. Fraction 2 contained a mixture of plain synaptic vesicles and flocculent material. Fraction 3 contained flocculent material only. Fractions 1 and 3 were partially purified by passage through a Sephadex column. Fraction 3 contained no shell fragments but contained finer flocculent material which, it is suggested, is composed of unit particles either occurring singly or linked together into chainlike or amorphous aggregates. Each unit particle appears to have four subunits and is here referred to as a tetrasome. Tetrasomes sometimes appear to be attached to the surfaces of the plain synaptic vesicles. Also, it is possible that aggregates of tetrasomes form part of the structure of the presynaptic dense projections.

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