1. A technique has been described for the preparation of clots from purified fibrinogen and thrombin of bovine origin which are suitable for study with the electron microscope. Experiments have been carried out to compare the fine structure of clots prepared at various values of pH.

2. The clots are composed of meshworks of single and compound fibers. At pH 8.5 the unit fibers have a smaller average diameter than those formed at pH 7.6 or pH 6.3. The tendency for the lateral association of unit fibers into compound fibers is markedly increased as the pH is decreased.

3. A striking feature of all the clots studied is cross-striation of the unit fibers. The periodicity of these striae is quite constant throughout (approximately 250 Å). There is a precise coincidence of the striations of the individual unit fibers where these are associated side by side to form compound fibers.

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