Abstract 1. 1.|Friend erythroleukemia cells (FELC, a differentiating cell line) were heated at various temperatures and heating sequences. Heat treatments which ranged from 41.0 to 45.0°C and did not cause differentiation in FELC and inhibited the differentiation response to DMSO in FELC. 2. 2.|Heating resulted in cell killing which increased with temperature and heating time. Protracted low temperature heating (40.0–42.0°C) or incubation at 37°C between two heat treatments at 45.0°C resulted in thermotolerance for both the endpoints of cell killing and differentiation. 3. 3.|High temperature heating (45.0°C) before heating at 41.0–42.0°C resulted in increased thermal sensitivity to the latter heat treatments. This was observed for both the survival and differentiation endpoints. 4. 4.|A comparison was made of the thermal sensitivity for the two endpoints of cell killing and differentiation.