1. Erythrocytes are known to haemolyse in vitro at 48–50°C. We hypothesized that erythrocytes might be damaged at much lower temperatures if they are incubated for prolonged periods. Erythrocytes from healthy human donors ( n = 7) were incubated at 37, 40, 42, 44, 46 or 48°C for 4–48 h. The haemolytic percentage and osmotic fragility were then measured by a modification of the method of Parpart et al. . 2. Significant haemolysis and increased fragility were not observed at any temperatures after incubation for 4 h. However, the haemolytic percentage increased after incubation for 24 h at 44°C (9.1 ± 4.9%, P <0.01), 46°C (52.4 ± 14.1%, P <0.01) and 48°C (98.0 ± 2.6%, P > 0.01) and after incubation for 48 h at 42°C (9.8 ± 4.5%, P <0.01) when compared with the values before heating (1.1 ± 0.9%). The osmotic fragility also increased after incubation for 24 h at and above 42°C. 3. Although heat-induced haemolysis and an increase in fragility have not been known to occur below 48°C, these were quite apparent after incubation for 24–48 h at only 42°C. This suggests that with regard to thermal effects, it is important to consider not only the temperature but also the duration of heating.