Electron images can be used to provide amplitudes and phases for the structural determination of biological specimens. Radiation damage limits the amount of structural information retrievable by computer processing. A 400-kV electron microscope was used to investigate radiation damage effects on frozen, hydrated actin bundles kept at -168 degrees C. The quality of phases within and among images in a damage series was evaluated quantitatively out to 16 A resolution. It was found that the phases of structure factors with good signal-to-noise ratio (IQ less than or equal to 4) can be reliably retrieved from images taken at a cumulative dose of at least 25 electrons/A2.