1. The distribution of isotope following a single injection of either Ca⁴⁵ or C¹⁴-proline has been studied in young rats in which one tibia had previously been removed, killed and reimplanted.

2. The dead tibia took up about 25 per cent as much Ca⁴⁵ or C¹⁴ as did the living tibia and the possible processes by which this occurred are discussed.

3. Determination of the "accretion rate " by kinetic analysis of the Ca⁴⁵ data showed that this was much too high unless the physico-chemical process of uptake of isotope by bone was taken into account.

4. Under the conditions of the experiment it was not possible to estimate the rate of bone matrix formation using C¹⁴-proline as a tracer.

1. The radiographs of paired living and dead rat tibiae, obtained in an experiment previously reported, have been examined by densitometry.

2. The dead bone became progressively less dense than the living bone as the duration of the implantation increased.

3. The change in density was related to the quantity, but not to the quality, of the bone tissue examined.

1. Some physical properties of living and dead bone have been studied in rats; most of these are interrelated and ultimately depend upon the composition of the tissue.

2. Dead bone, remaining within the body, does not take up measurable amounts of mineral from the tissue fluid but retains its original physical properties of radiographic density, specific gravity, strength and composition.

3. The altered radiographic density of avascular bone seen in clinical practice is almost certainly relative unless there has been concomitant appositional new bone formation.

4. Some other explanation must be sought for the finding that dead bone takes up significant amounts of bone-seeking isotopes in radioactive tracer studies.

1. Homografts of eighteen-day-old foetal femora in pure strains of mice showed no fundamental difference in behaviour from grafts of more mature bone and cartilage.

2. Growth of bone was limited to a short period after transplantation and was abolished by previous immunisation. Cartilage growth alone was responsible for the increase in size of these transplants and did not appear to be influenced by the presence of immunity.

3. There is no reason to suppose that bone from an immature source is likely to behave more favourably than more mature bone homografts in clinical use.

4. The limited growth of cartilage and the total failure of bone survival in the heterografts indicate an immune reaction ofa different order from that which develops against the homograft.

5. The different effect of the homograft immune reaction on cartilage and bone enabled certain conclusions to be drawn concerning the part played by these two tissues in determining the form of a bone. Cartilage growth and development is shown to be regulated in large part by intrinsic factors. Bone growth and form on the other hand is shown to be dependent largely upon extrinsic influences.