1. Following intramuscular injection into rabbits of alcohol alone, twenty-three of thirty-seven triceps (62 per cent.) and twelve of forty quadriceps (30 per cent.) contained cartilage or bone, alone or in combination.
2. Two of twenty-seven triceps and seven of twenty-nine quadriceps contained cartilage or bone, or both, following injection of calcium chloride.
3. There is a greater tendency for pre-cartilage, cartilage, and bone to form near the origin or the insertions of muscles than in the mid-portions.
4. Younger animals (one month old) formed larger amounts of more mature cartilage and bone than older rabbits, following injection of alcohol alone or of calcium chloride.
5. There was no appreciable difference in the frequency of bone and cartilage on the basis of sex.
6. Diethylstilbestrol, in the dosages used, did not influence the course of the intramuscular reactions.
7. The incidence and extent of intramuscular cartilage and bone was increased by exercising the animals.
8. Intramuscular cartilage and bone formation was not observed if suppuration occurred as a result of the injection of the irritant solutions.