Major discrepancies exist between both the degree (or cholesterol excess limit) and duration of metastable cholesterol supersaturation in concentrated model biles and native human gallbladder bile. The comparatively reduced metastability of in vitro model systems suggests that a stabilizing factor is absent in these more concentrated systems. In contrast, the metastable supersaturation behavior of the dilute model and hepatic biles is similar particularly with respect to their markedly prolonged metastability, i.e., nucleation time. These observations suggest that changes in the lipid component alone (e.g., structural alterations resulting from dilution) could account for the similarity. The structural component in hepatic biles and in comparable dilute model systems appears to be small unilamellar vesicles which have an unusual duration of metastability and a high capacity for cholesterol transport. In this sense, metastability properties characteristic of pure micellar systems are neither observed nor to be expected in human hepatic bile and are only partially responsible for metastability in gallbladder bile.