A general saturation equation is derived which is shown to describe a wide variety of nutrient-response relationships in higher organisms. Iterative multiple linear regression analysis is used to obtain least squares estimates of the constants defining theoretical nutrient-response curves. Curves thus generated accurately predict experimentally observed responses. From this treatment, response parameters are developed which are analogous to Vmax and Km of enzyme kinetics. It is proposed that this model be applied in evaluating nutritional requirements and in assessing the relative biological efficiency of nutrient sources.