1. Specimens of human duodenal mucosa were obtained at duodenotomy. Superficial mucosal scrapings were homogenized in isotonic sucrose solution and fractionated by differential centrifugation. The distribution of organelles among the subcellular fractions was monitored by assay of suitable marker enzymes.

2. Enterokinase was recovered predominantly in the nuclear + brush-border fraction and 80% of the total activity was found to be particulate; approximately 20% of the enzyme was present in the soluble fraction, compared with 1% of the brush-border markers sucrase and alkaline phosphatase.

3. The brush-border-containing fraction was subfractionated by treatment with hypertonic Tris followed by differential and density gradient centrifugation. Enterokinase was distributed among the subfractions in parallel with brush-border markers and was concentrated in a subfraction which was highly enriched in microvillous membranes.

4. It was concluded that enterokinase is localized primarily to the microvillous membrane of the epithelial cell brush border in man, but that in addition a proportion of the enzyme may be present in a soluble or easily released form in the duodenal mucosa.

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