HSP72 levels in the cellular and the nuclear (TX-insoluble) fraction before and after heating of heat- and sodium arsenite-induced thermotolerant and non-tolerant HeLa S3 cells have been investigated by 1D- and 2D-electrophoresis, followed by Western blotting and immunostaining, using a newly developed monoclonal antibody that specifically detects HSP72 (Heine et al. 1991). HSP72 was constitutively expressed in HeLa S3 cells and elevated upon heat or arsenite stress. Immediate association of HSP72 with the nuclear fraction was induced by heat but not arsenite. However, at the time of maximal thermotolerance, elevated levels of HSP72 were found associated with nuclei isolated from both heat- and arsenite-induced thermotolerant cells. After (test) heat treatments (0-60 min at 45 degrees C) translocation of HSP72 to the nuclear fraction in all cells was observed, albeit with different kinetics and to different plateau values. When tolerant and non-tolerant cells were allowed to recover from a heat stress (at 37 degrees C) before isolation of the nuclei, no dissociation of HSP72 from the nuclear fraction was observed within a 5 h time period. Our data indicate that association/dissociation of HSP72 with/from the nuclear fraction is not related to the recovery from heat-induced intranuclear protein aggregation (Kampinga et al. 1992), nor to the extent of thermotolerance in the human HeLa S3 cell line.