Salmonella enteritidis was injected into isolated "loops" of duodenum, midgut and ileocaeca in anaesthetized 1-day-old chicks. Using light microscopy with conventional staining and fluorescent antibody techniques, the organism were observed in increasing numbers in the epithelium and lamina propria of each region between 2 and 12 h after inoculation. The potential ability of the salmonellas to enter the mucosa at any level of the intestine was thus demonstrated. The penetration of caecal epithelium by the organisms was studied by electron-microscopy. Uptake of salmonellas was associated with evaginations which formed at the luminal surface of the epithelium. In the epithelial cells, the bacteria were enclosed within membrane-bound vacuoles and appeared undamaged by intracellular passage. Surface evaginations almost completely replaced the brush border in regions where large numbers of the organisms were involved. Occasional salmonellas were found in the lamina propria both free and intracellularly. Results of culture and light microscopy following oral and intracloacal inoculation are also presented.