A preclinical investigation has been made of the effects of microwave heating in enhancing the response of pig skin to X-rays. Areas on the flanks of Large White English pigs were treated with X-rays alone or with X-rays followed by heating at 42°C for 1.5 hours. The treatment was given as a single dose or as three fractions over six days. Heating was produced with a microwave direct-contact applicator operating at 2.45 GHz. The early skin reaction was scored quantitatively by an arbitrary numerical system. For all treatments, microwave heating was shown to have little effect in enhancing the response of pig skin to ionizing radiation.