Human skin, and its isolated cells, respond to insults with a variety of repair and protective mechanisms. One such mechanism is the production of heat shock proteins (HSPs). Heat shock proteins help the other cellular proteins fold correctly into their active three-dimensional structures. Therefore, they can enhance the survival of cells under harsh, denaturing conditions. In order to develop a means of promoting the heat shock response to prepare the skin to withstand insult, we are investigating materials that appear to protect the skin biologically. One such material is vitamin D3 and its precursors. We have observed that keratinocytes treated with 7-dehydrocholesterol (7-DHC), a precursor of vitamin D3, have increased levels of protein and mRNA for heat shock proteins. In addition, we observed that topically applied 7-DHC increases the minimal dose of UVB required to induce erythema. These data suggest that 7-DHC can induce heat shock proteins in skin keratinocytes and that they will be more resistant to UVB insult.