Hyperthermia has been shown to inhibit the activity of DNA polymerase-beta when either the intact Chinese hamster cells or partially purified enzyme samples from CHO cells are heated (42.2 or 45.5 degrees C). However, the loss of activity from heating isolated enzyme samples can be either greater or less than the loss from heating intact cells. For example, treating cells with the membrane-active agent procaine-HCl greatly sensitizes the cells to heat-induced loss of enzyme activity but has no effect on the heat sensitivity of isolated enzyme. Furthermore, the heat sensitivity of the isolated enzyme depends greatly on the purification steps and can be reduced by heating the enzyme in the presence of bovine serum albumin, activated DNA, or Langendorf salts. These observations, considered in relation to others in the literature, suggest that heat inactivation of polymerase-beta occurs from heat-induced changes in the intracellular environment, which in turn modify the direct thermal denaturation of the enzyme within the cell.