The prebiotic IMO, the prebiotic PrimaLac, and their combination as a symbiotic were unable to further reduce the yolk cholesterol contents of hens at 32 or 36 wk of age in the present study, possibly due to natural selection pressure to maintain a certain level of cholesterol in the egg for use by the developing embryo, which would ensure that a minimum physiological level is reached (Hargis 1988). Furthermore, Noble and others (1990) reported that egg lipids were not successfully reduced in eggs of laying hens due to the mechanism of laying hens that ensures that the lipid supply for yolk formation is protected from temporary dietary and environmental imbalances.