The tarso-metatarsal distance in chicks, measured radiographically, was used as a criterion in a curative method for Vit. D3 assay. The slope of the linear response curve (tmt distance[long dash]log vit. intake) depends on the diet. A diet giving a satisfactory slope contained ground yellow corn 34.4, middlings 28.4, dried skim milk 9.4, dried grass meal 5.5, soya flour 15.4, bone meal 4.4, Iodised salt 1.0, charcoal 0.5, arachis oil 1.0. Responses were linear in the dose range 10 to 30 units of Vit. D3/100g. diet. On this diet gain in wt. may be used as a criterion of cure with equal reliability to tmt distance. The test period is 5 wks. and with 20 chicks/group the assay value obtained possesses fiducial limits (P = 0.95) of 78-125%.