Rumens of fistulated cattle were manually emptied, washed with a buffered saline solution, and then filled with buffered saline with or without added antibiotic. After 3 days without feed, cattle were slaughtered and blocks of rumen epithelium were assayed for urease and prepared for scanning electron microscopy. Urease activity in treated animals was less than 3% of that in controls, thus indicating almost complete removal of urease-producing bacteria, and scanning electron microscopy showed a dramatic decrease in the number of adherent bacteria. The technique can thus be used to assess the relative contributions of epithelium and adherent bacteria to various processes known to occur at the epithelial boundary.