The histology of the organs may be described together. The infiltrate described is a densely packed mass of large cells with large, well staining nuclei and a very narrow rim of protoplasm. It is not limited by any definite wall or septa. It has no interstitial tissue. There is no blood supply in the densest masses but the walls of the blood vessels remain intact wherever the mass surrounds them. In the lungs it has involved all structures indiscriminately, and has destroyed practically all of the respiratory surface. It seems to follow by preference the peribronchial space. A few glands may be seen in the centre of this mass, but they are rapidly undergoing degeneration. In the liver the infiltrate is chiefly beneath the capsule extending inward but a very short distance. There are no large masses as in other organs but small infiltrates are seen at the portal areas. The spleen shows a diffuse excess of pulp cells and many of the cells above described, the difference being only in the size of the nucleus which is smaller in the pulp cells. There are very small round cells relatively. Follicles are absent, connective tissue not altered. Note states follicles prominent; this is due to nodal hyperplasia of the large mononuclears above described. The parenchyma in the kidney is anemic, the epithelium is slightly pigmented but this is probably not abnormal. Between the tubules especially of the outer layer of the medulla and medullary ray but also in the cortex and around the glomeruli are diffuse, irregular, infiltrating masses of the cells as described above. In some places in the kidney hyaline casts are being formed probably due to the degeneration of the epithelium by pressure. There are a few distentions of the tubules. The capsular space is free. In the neighborhood of the collections the capsules are a trifle thicker than normal. Lymph nodes, similar to spleen in that most of the bodies are thoroughly overrun with the large mononuclears. The sinuses, both marginal and internal, are practically obliterated by these cells. In the blood vessels of the lungs and liver there are many large mononuclears, perhaps not as large a number as might be seen in leucemia, but decidedly in excess of normal. Common Marmoset (_Callithrix jacchus_). ♂ Adult. Had cage paralysis for two months before death and declined gradually from that time.