The purpose of this study was to determine whether the addition of high levels of yeast to casein and soy diets could modify the well known effects of any of these proteins on plasma cholesterol. Rabbits, were fed either a diet containing soybean protein-brewer's yeast or casein-brewer's yeast (each protein source providing 50 percent of the dietary nitrogen content) and casein and soybean protein basal diets. Brewer's yeast was obtained from a local beer factory in its non-debittered form. The diets contained 20 percent protein, 9 percent coconut oil and 1 percent corn oil, with no added cholesterol. After a 22 day experimental period, rabbits fed casein developed hypercholesterolemia whereas those fed the soybean protein diet did not. The replacement of 50 percent of the soy nitrogen by brewer's yeast nitrogen, increased the total cholesterol plasma level, but significant differences were only observed between rabbits fed casein and casein-yeast and those fed soybean protein. No differences in high density lipoprotein cholesterol could be detected among the groups. However, the HDL-cholesterol/total cholesterol ratio was significantly reduced in response to soy substitution by brewer's yeast. The (low density lipoprotein + very low density lipoprotein)--cholesterol was increased in all groups with the exception of the animals fed purely soy protein. These data suggest a hypercholesterolemic activity of the dietary non-debittered brewer's yeast. Nevertheless, according to the amino acid composition, the factor responsible for the reported effects of dietary yeast was not associated with a high lysine to arginine ratio which could be due to extracellular components.