RIF-1 tumors (100-300 mg) were exposed in vivo to heat treatment (41-48 degrees C) for 30 min and then assayed for either cell survival or tumor control. The tumors were heated either with normal perfusion or with temporary vascular occlusion (clamped for 30 min prior to and during the 30-min treatment). The physical technique of water bath heating ensured temperature uniformity in both the perfused and vascularly occluded tumors. Survival curves for tumor cells heated under both conditions had a shoulder and exponential regions. While the T0's were not statistically different in the two cases, cells from the tumors whose blood flow had been occluded showed an enhanced sensitivity to heat as evidenced by a reduction of the shoulder by 2.5 degrees C. A similar increase in sensitivity was measured with the tumor cure assay with the TCT50 decreasing from 47 degrees C for unclamped tumors to 45 degrees C for clamped tumors. The two assays are therefore in excellent agreement in assessing the effectiveness of heat treatment and the influence of vascular occlusion on the heat sensitivity of this tumor. Since the clonogenic assay was performed immediately after treatment, this agreement between assays indicates that direct cell kill by heat is the major factor in determining cure in this tumor.