How fatty acids stimulate enteroendocrine cells to release cholecystokinin (CCK) is largely unknown. Recently, we proposed that the murine enteroendocrine cell line, STC-1, responds to insoluble fatty acid aggregates rather than fatty acid monomers in solution. This hypothesis led to two testable predictions. First, other insoluble particles of similar size but unrelated to fatty acid may be able to stimulate STC-1 cells in a similar fashion to dodecanoic acid and second, fatty acid sensing in STC-1 cells should be fairly insensitive to chemical modifications of the fatty acid as long as these modifications do not greatly alter the ability of the molecule to form insoluble aggregates. We used several analogues of dodecanoic acid and several varieties of latex microsphere (varying in size and surface charge) to see whether the predictions of our model hold. We found that while there was at least one latex microsphere that could induce CCK secretion and calcium mobilisation in STC-1 cells, there was a very poor correlation between the presence of insoluble aggregates and a cellular response. Instead the most important property, determining the potency of fatty acid analogues as stimulants of CCK secretion, was their amphipathicity. Removal of either the polar head or lipophilic tail completely abolished the ability of a given fatty acid analogue to stimulate STC-1 cells. These data suggested that while fatty acids can stimulate cells as aggregates, they may also be acting in monomeric form with the oil:water partitioning coefficient playing a crucial role. We finally resolved this issue with the observation that the sulphate ion greatly altered the response of STC-1 cells to monomeric dodecanoic acid. In the presence of sulphate, STC-1 cells will only respond to dodecanoic acid aggregates whereas when sulphate is replaced with chloride the cells clearly respond to dodecanoic acid monomers which are completely in solution. In summary we propose that dodecanoic acid can stimulate STC-1 cells via two separate pathways one involving fatty acid monomers in solution and one involving fatty acid aggregates. Which pathway dominates depends on the presence of sulphate in the extracellular medium.