Photon correlation spectroscopy has recently been shown to be a fast and accurate method of measuring the translational diffusion constant (D) of macromolecules. The measurement time is so low that it is possible to follow changes in the diffusion constant as a function of temperature, pH, etc. Most of the photon correlators in existence employ an electronic autocorrelator which relies on a clipping process to construct the desired functions. The spectrometer which we describe here is on-line to a Digital PDP-9 computer which is programmed to carry out a complete correlation analysis. Advantages and disadvantages of this technique are discussed.