1. Three groups of one month old rats were fed for a period of four to five weeks on a rachitogenic diet (group R), the same rachitogenic diet with vitamin D (group N), and a complete diet (group S).

2. Young's modulus of elasticity E for bone can be derived from measurements of the deflexion of the centre of a femur loaded at the centre and supported at its ends.

3. The three different diets produced significant differences in breaking stress S_B, Young's modulus E, and percentage ash in the bones. It has not been shown conclusively that higher ash content alone is responsible for the greater S_B and E values of bones produced on the better diets.

4. The value of E in group R was 0·6 x 10⁶ lb./in.²; in group N 1·0 x 10⁶ lb./in.²; and in group S (which can be taken as normal) 1·6 x 10⁶ lb./in.².

5. There is a high correlation between S_B and E even when the effect of diet is eliminated.

6. Although the bones produced on the good diet (group S) were much stronger than those of groups N or R, the strain at the elastic limit was the same (about 1·5 per cent.). The strain at rupture tended to be higher in groups N and R than in group S.

7. The properties of bone as a structural material are discussed.