It has not been fully understood how intact protein antigens escape digestion in the course of absorption. The present study was designed to investigate the mechanism that heat stress induced an increase in intact protein antigen absorption. Human colonic cell line Caco-2 cells were treated with high temperature (37 to 43°C) for 60 min. Epithelial permeability was evaluated by horseradish peroxidase (HRP) flux and dextran flux. Activity of the major intracellular proteolytic enzyme, acid phosphatase, in Caco-2 cells was determined. HRP products in Caco-2 cells were observed by electron microscopy (EM) and analyzed with a computerized image processing system. Heat stress significantly increased intact protein HRP transport across Caco-2 monolayers, decreased acid phosphatase activity of the cells, and significantly reduced transepithelial electric resistance of Caco-2 cells. EM results showed that HRP transport across Caco-2 monolayers occurred mainly via the intracellular pathway.