Tissue culture cell line MDCK grew with the histologic characteristics of a papillary adenocarcinoma when cultured in vitro in a three‐dimensional matrix of collagen‐coated cellulose spongeor when inoculated in the chick embryo. When suspensions of cells were injected intravenously in the chicknumerous implants were seen in the brain one week later. Transplanted to the chorio‐allantois with fragments of embryonic heartMDCK was found in the cardiac mesenchyme and within the lumens of many vessels of the transplanted heart. MDCK also exhibited the physiologic characteristics of renal tubular epithelium. Cultured on a glass surfaceMDCK formed a monolayer of epithelium interspersed with numerous blisterseach of which was composed of many cells. The walls of the blisters and the contiguous monolayers on glass were made up of polarized epithelium with microvilli on the medium‐bathed surface. Time lapse movies demonstrated that the blisters were in active fluid transport. Low concentrations of ouabain abolished the blistering overnightsuggesting inhibition of an ATPase essential for the sodium‐potassium pump associated with the plasma membrane.