Abstract— Treatments such as plating at elavated temperature, plating in the presence of pantoyl lactone, plating on minimal medium, plating on complete medium after intense white illumination, or plating on complete medium after holding in buffer, are known to increase survival of U.V. irradiated Lon strains of Escherichia coli. The effect of these treatments on Hcr Lon+ and Hcr Lon strains was studied. Pantoyl lactone and elavated temperature recovery seem specifically related to the Lon defect. Holding in buffer does not enhance survival in an Hcr strain, and only exerts a small effect in an Hcr Lon strain. White irradiation and minimal medium may greatly enhance recovery of both Hcr and Lon strains. However, an Hcr Lon strain is sensitive to white irradiation itself. Copyright © 1969, Wiley Blackwell. All rights reserved