The preparation of crystals of bovine liver catalase, and their use as an internal standard of length in electron microscopy, are described. Absolute sizes in the magnification range x 10,000 to x 150,000 may be determined simply and routinely with an accuracy of ± 2.5 %, and relative sizes with an accuracy of 1.5 %. Measurement of the lattice spacings of crystalline catalase by low-angle electron diffraction is described. Results of other techniques are discussed in relation to this measurement. A critical appraisal of all available data suggests a figure of 172±4 A for the principal catalase lattice spacing. Lattice lines separated by half this distance are used in actual size determinations.