Alterations were recorded from capillaries and connective tissue by means of optical light and electron microscopy in investigations of subepithelial structures, following experimental Escherichia coli infection. These alterations led to an impairment of the blood-udder barrier and thus to increased extravasation of cells (especially neutrophilic granulocytes). They were found to accumulate in subepithelial connective tissue and, eventually, to penetrate in high quantity the epithelium proper, and, consequently, to be considerably involved in alterations to the latter.