In many animal production experiments, carcass weight is used as the sole criterion for the effectiveness of the treatments and of the nutritional status of the animals involved. In these experiments some knowledge of the carcass components would be desirable in order to assess any effects on the various tissues. Determination of these components has hitherto been costly and time consuming. Accordingly, these disadvantages have lead to a search for simple indices of carcass composition which can be readily applied even in relatively large-scale experiments. Until recently, however, no research worker has specified a relationship between carcass weight and carcass composition which would justify the use of carcass weight alone. There are, of course, many indications which suggest that such a relationship would exist, at least, for the main tissues of the carcass.