Studies were carried out with HeLa S-3 cells to determine whether exposure of the cells to "differentiating agents" would modify the thermosensitivity of cells under the cellular conditions where there is no significant perturbation of the cell cycle distribution. Sodium butyrate, dibutyryl cyclic adenosine 3':5'-monophosphate (dbcAMP), and retinoic acid all afforded pronounced protective effects on the thermosensitivity of HeLa cells. The kinetics of expression of thermal resistance induced by the agents varied with different agents. Sodium butyrate (1 mM) was only capable of inducing the thermal resistance during the time of heating (42 degrees), while a minimum 8-hr exposure to dbcAMP (1 mM) and 48-hr exposure to retinoic acid (10 microM) prior to heating at 42 degrees were required to demonstrate the thermal resistance. The presence of dbcAMP and retinoic acid was not required during the heating. The thermal resistance acquired by the exposure of cells to dbcAMP and retinoic acid was reversible following the removal of the drugs with a time course varying with the duration of preexposure time. This difference in the kinetics of thermal resistance suggests that these agents may have a different mode of action for the induction of thermal resistance in HeLa cells.