Immunotherapy, i.e. stimulation of the body's immune response against tumor cells, is a promising approach in cancer treatment. In this context, heat shock proteins (HSP) have been shown to function in tumor antigen chaperoning. HSP are evolutionarily conserved and show increased expression in response to chemical and physical stress. Two members of the HSP family, HSP 70 and 90, seem to further act as immunostimulating agents because of their possible involvement in tumor antigen presentation. We cultured the human hepatocellular carcinoma cell line HepG2 and investigated its HSP content under normal and hyperthermic conditions. Flow cytometry showed increased levels of HSP 70 and 90 after heat shock at 41.8 degrees C for 60 minutes, measured after a subsequent incubation time of five hours, as compared to untreated cells in vitro. We further observed a clear correlation between the HSP 70 and 90 levels and the necrotic cell subpopulation in heat shocked tumor cells. We conclude that HSP expression in HepG2 cells can be enhanced by heat shock treatment in vitro. We suggest that this mechanism can be exploited in increasing tumor immunogenicity.