It has been shown by us in previous papers1 that the actual cholesterol content in the bile is not so important in gallstone formation as the ratio of the cholesterol to the substance that holds it in solution, that is, the bile salts. Therefore, it becomes important to study the factors influencing this ratio. The single factor of the concentration of the cholesterol and of the bile salts taken separately have each received much previous attention, and there is a large literature on the subject which it is not necessary to review here. However, the important factor of simultaneous estimation of the two substances has rarely been employed, and the following studies are an attempt to fill this gap. This paper will be mostly concerned with the cholesterol factor in this ratio and with attempts to vary bile cholesterol by the administration of fat or lipoids. The literature on