As a preliminary step in a study of the origin of cells in the lesions following experimental inoculation of mice with Line I transmissible leukemia, the cellular component of lesions was studied. A large cell type of the lymphatic series predominated. The chromosome number determined was 40, the normal diploid number for mice. Mitosis was regular except for rare aberrancies found in degenerative lesions. Cytological methods used did not differentiate between the cell type predominating in lesions and a similar morphological type found in normal mouse tissue.