Experimental and clinical evidence suggests that elevated tissue temperature enhances the susceptibility of tumors and other tissues to ionizing radiation, as reviewed extensively by Selawry, Carlson, and Moore.7 Recently, Crile has confirmed some of these earlier observations and has obtained encouraging results from the application of this principle to 3 patients with malignant tumors.1 Nearly all reported studies have employed isolated single exposures of the body or tissue to heat. Heat alone has been shown by Selawry and co-workers to have an inhibitory effect on tumor growth,6 but as with any other attempt to induce tumor regression, the effect on normal tissues is the limiting factor in its application. To attempt to exploit the inhibitory effect of heat more fully by delivering a greater total heat exposure, this study was undertaken to evaluate the effects of intermittently induced fever on the "take" of the Walker 256 carcinoma in