Normal and thermotolerant H35 cells were submitted to step-down heating (SDH). SDH can significantly reduce the induction and expression of thermotolerance. For SDH a sensitizing treatment (ST) at 44.6 degrees C was followed by a test treatment (TT) at a lower hyperthermic temperature. The comparison between the thermotolerant and non-thermotolerant condition was based on isosurvival ST doses. For both conditions dose-effect relationships were obtained by plotting the ST-surviving fraction against the D0 of a TT. The TT was at either 41 or 42.5 degrees C, representing respectively, a permissive or a non-permissive condition for chronic induction of thermotolerance (CIT). The complex dose-effect relationships are partly exponential. In non-thermotolerant cells tested at 42.5 degrees C the dose-effect relationship between ST and TT is relatively weak. At 41 degrees C, however, the expression of CIT is strongly inhibited after a ST that kills < 20% of the cells. At higher ST doses the response is comparable with that at 42.5 degrees C. In thermotolerant cells a high degree of thermosensitization is also observed for relatively low ST doses, but in contrast with non-thermotolerant cells a stronger dose-effect relationship remains at the higher ST doses. Ultimately this results in a comparatively higher degree of thermosensitization that can be achieved in non-thermotolerant cells. For example, at an isosurviving fraction of 0.15 the reduction of D0 is non-thermotolerant cells at 42.5 degrees C is less than five times, whereas in thermotolerant cells, the D0 reduction is between 40 and 50 times. A similar reduction is found in non-thermotolerant cells tested at 41 degrees C. Subsequently, an isosurvival ST dose of about 40% was used in combination with a TT that was varied between 39 and 44 degrees C. D0's were plotted in an Arrhenius diagram to obtain a time-temperature relationship for the effect of SDH on thermotolerant and non-thermotolerant cells. The four plots are all biphasic with a downward inflection. Thermotolerance causes an upward shift of the inflection point of 2 degrees C relative to single-heated cells, whereas SDH causes a downward shift of 1 degree C in single-heated cells and of 2 degrees C in thermotolerant cells. For most of the temperature range, i.e. 39-43.5 degrees C, SDH decreases the activation energies.