Abstract The molecular weight of crystalline a-chymotrypsinogen has been determined from amino acid analysis, light-scattering, and sedimentation-diffusion measurements. The values obtained by the use of these methods are, respectively, 25,100, 26,000 and 24,200. These values, together with the recently reported X-ray estimate of 25,000, converge toward 25,000 as the most probable molecular weight of a-chymotrypsinogen.