Size distributions of PHB granules synthesized in recombinant Escherichia coli are determined by photosedimentation. Mean granule Stokes diameters are in the range 1.13-1.25 mu m, which is larger than reported values for wild type microorganisms. Treatment with 1.5% hypochlorite and mild centrifugation did not affect granule size distribution. Treatment with 10% hypochlorite led to a significant reduction in mean diameter and total PHB.