The technique of digital autocorrelation of intensity fluctuations in scattered laser light has only recently been applied to biological problems. It affords precise and rapid measurements of the translational diffusion constants of macromolecules; typically, an accuracy of 1 % can be achieved in about 1 min. This paper is intended to be sufficiently detailed and to contain sufficient information to enable workers in the field of biophysical chemistry to assess the applicability of the technique to problems of their interest. We discuss the origin of intensity fluctuations in laser light scattered by a macromolecular solution and give a brief review of the theory of photocount autocorrelation. We then describe the apparatus, and the methods for the reduction of experimental data. In addition to discussing the least-squares data analysis, we present a general method by which various spurious effects such as dust in the sample, and real effects, such as macromolecular polydispersity, may be detected and characterized. Finally we consider sources of error in the measurement of D and present typical data for solutions of R17 virus to illustrate the performance of our apparatus. © 1974, American Chemical Society. All rights reserved.