The hyperthermic sensitivity of cultured human glioblastoma cells was determined in 1 degree increments from 42 through 46 degrees over the pH range of 6.1 to 7.4. These cells were 5 to 10 times less heat sensitive than were Chinese hamster ovary cells and are among the most resistant cells examined in cell culture. At pH 7.4, the rate of cell killing doubles with each 1 degree increase in temperature above 44 degrees, a phenomenon previously observed in other cell lines heated above 43 degrees. It is speculated that thermotolerance develops during heat treatment and accounts for this apparent resistance to heat below 44 degrees. Reduction in medium pH from 7.4 to 7.0 during heat treatment did not significantly influence thermal sensitivity; however, below pH 7.0 a pH-sensitizing effect was increasingly evident. The magnitude of this sensitizing effect, determined from survival curve slope ratios, was temperature dependent and was most pronounced at temperatures which were moderately lethal at pH 7.4. At pH 6.7, heat sensitivity was increased from a factor of congruent to 1.25 at 46-44 degrees to 2 at 43 degrees compared to treatment at pH 7.4. This pH-sensitizing effect and its temperature dependency was more pronounced at pH 6.1.