1. Analytical subcellular fractionation techniques have been applied to human jejunal homogenates to study the localization of serotonin and motilin.

2. Serotonin, detected by a fluorimetric assay, showed two distinct particulate distributions in the density gradients with modal densities of 1.14 and 1.20. Motilin, whether detected by N-terminal or C-terminal specific antibodies showed similar density distributions with a modal density of 1.20, coinciding with the denser peak of serotonin.

3. These results lend support to the histochemical suggestions that motilin is located in a serotonin-containing granule.

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