The design and construction of a laser interferometer using the method of exact fractions for high-accuracy calibration of gauge blocks up to about one meter in length, are described. The interferometer is based on the classical Michelson arrangement. A two-mode stabilized He-Ne laser emitting light at a wavelength of 633 nm is the primary wavelength reference for the interferometric length measurement, since its frequency is calibrated by heterodyne measurement with an iodine-stabilized He-Ne laser at 633 nm which is used to maintain the national meter standard. A second two-mode stabilized He-Ne laser, at a wavelength of 543 nm, is used in order define the absolute length. The estimated standard uncertainty for interferometric length measurements is about 50 nm for a 1 meter gauge block. Results from trial measurements made on various gauge blocks, including a bilateral comparison, are reported.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.