Plutonium, thermally cycled through the low-temperature allotropic transformations, exhibited extensive physical damage. The physical damage was greater than that reported for any other metal or alloy. The extent of physical damage was determined by measurements of fluid displacement and the dimensional changes. The internal porosity was examined metallographically. Physical damage varied considerably, depending on the cycling conditions and the characteristics of the metal (particularly inclusions and casting imperfections). Observations showed that increasing the specimen size produced a greater decrease in density as a function of the number of cycles. The degree of void formation was directly proportional to volume change associated with the phase transformation. Other variables such as the length-diameter ratio (constant diameter) had little or no influence on the amount of physical damage. The dimensional growth was both radial and longitudinal. The tensile strength and the yield strength of tensile specimens cycled ten times between the gamnna and alpha phases were decreased to an average of 30,000 and 26,000 psi, respectively. (auth)