1. Severe osteolathyrism has been induced in chicks of different ages by a diet containing 50 per cent seeds of Lathyrus odoratus.

2. In these chicks, most of which became paraplegic after seven days, a meningeal tumour, articular and bony deformities, spontaneous fractures and osteoporosis have been observed.

3. In cartilage the lesion involves depletion of both neutral and acidic polysaccharides.

4. The primary effect on bone consists of changes in the osteoblasts and in the osteocytes involving cytoplasmic granulation and vacuolation, "mineralisation" and eventual disintegration of the cells.

5. These events are followed by osteoporosis. abnormal mineral deposits in the marrow spaces and reactive callus-like activation of the periosteum.

6. The role of the osteoblast and of the osteocyte in mineral transit and deposition is reconsidered.