The virus particle observed in the leukemic material is round or ovoid in shape and has a mean diameter of 870 A. The main features of this virus are the concentric spherical membranes, the outer and the inner virus shells. Ultrathin sections of pellets prepared by ultracentrifugation of the leukemic filtrates have revealed the same type of particle identified in the liver, spleen, and bone marrow of the leukemic mice. The observations made in these tissues have been entirely superimposable up to now as far as virus-host cell relationships are concerned.

The specimens were fixed at different stages of the leukemic process from the 21st to the 78th day after inoculation with a cell-free leukemic filtrate. The virus was found in approximately 40 percent of the specimens taken from mice treated with leukemic filtrates. In this group of animals no preliminary study of the blood was made to indicate how generalized the disease was at the time the animals were killed. More recently, when specimens were taken only if the blood smear showed marked leukocytosis with many primitive cells and erythroblasts, particles appeared to be present in a considerably higher number of cases. Unfortunately, even in such material virus may be seen only in small numbers. This is possibly due to the choice of material, for it is conceivable that the virus reservoirs are located in other tissues or observable at different stages of the leukemia. Up to now, however, there is no clear relationship between the presence of the virus in the tissues and the stage of the leukemic process. All the viruses were found outside cells, in the intercellular spaces, or in intimate contact with the cell membranes. The virus was never observed in the cytoplasmic matrix, in the cytoplasmic organelles, or within nuclei.