1. In normal bone, microscopic hardness was found to be an accurate and reliable measure of the degree of mineralization. 2. The progression of mineralization that accompanies skeletal maturation was found to be reflected by an increase in the microscopic hardness. After the age of thirty, there was very little variability in the hardness of the diaphyseal cortex or of the ilium. 3. The hardness of cortical bone taken from selected sites within an individual varied widely, but there was very little variability in the hardness of the same bone taken from standard sites in different individuals. 4. The effect of osteoporosis, Paget's disease, rickets, and osteogenesis imperfecta on microhardness was tested on a small nunmber of individuals. 5. Microhardness testing is the only known technique for measuring the microscopic physical effects of variation in the composition of bone.