The in vivo immunological events in dextran sulfate sodium (DSS) -induced colitis were evaluated. Rats were fed water (control) or 5% DSS. Colonic sections were assessed by light microscopy, Gram stain, immunohistochemistry, and electron microscopy. A progressive decline in number and increase in fragmentation of bacteria in the colonic lumen was observed over time. Luminal bacteria were the first to show heat shock protein 60 (HSP60) staining (day 3). Macrophages in close proximity to these bacteria were next to show such staining (day 6), and finally the damaged epithelial cells when colitis became severe (day 15). Ultrastructural assessment showed cell-cell contact interactions between macrophages and dendritic gammadelta T cells. An increase in the number of gammadelta T cells and ED1-positive macrophages in the affected colonic tissue over time was documented. These results suggest colonic bacteria, host macrophages, and gammadelta T cells play specific roles in the immunological reactions in DSS-induced colitis, possibly via an HSP60-mediated mechanism.