The development of thermotolerance in L1A2 cells in vitro was investigated after fractionated hyperthermia at 42 degrees C. A single pretreatment of 90 min at 42 degrees C resulted in maximal thermotolerance at a 10 h fractionation interval with a thermotolerance ratio (TTR) of approximately 4.5. Thermotolerance was maintained at this level if the cells were exposed to 1-3 additional 90 min pretreatments separated by 10 h intervals. At a 6 h fractionation interval, where thermotolerance was still developing, additional 90 min pretreatments raised the level of thermotolerance to the level induced by the 10 h interval; the TTR increased from 2.1 to 4.5. Intervals of 20 and 24 h, at which there was decay but not complete disappearance of thermotolerance, did not induce such a build-up, and the TTR remained constant at 2.5 and 1.6, respectively. With 2 h fractionation intervals, where a single pretreatment did not induce thermotolerance, no thermotolerance was induced by additional pretreatments. With a shorter preheating time (e.g. 45 min at 42 degrees C) administration of a single pretreatment induced maximal thermotolerance after 6 h with a TTR of 3.0; additional 45 min pretreatments separated by 6 h intervals maintained this level. The thermotolerance induced by multifractioned hyperthermia as described above decayed in a similar way to that of cells exposed to a single pretreatment.