1. Ultrastructural studies were made of 400 agranulocytes, each from the thoracic duct effluent of 12 normal Holstein calves of both sexes.
2. Tabular electron microscopic evaluation of the agranulocytes present demonstrated that 89 per cent were lymphocytes, 4.8 per cent plasmacytes, 1.3 per cent reticular lymphocytes, 4 per cent proplasmacytes, and 0.7 per cent mitotic forms of the various cell types enumerated.
3. Mitochondrial tabular studies demonstrated that profile numbers (6.2-8.2) and profile sizes (0.15-0.25 µ2) were similar among cell sections of the five designated cell groups in the calf and the lymphocyte of the human. Monocyte mitochondrial profiles of the human were highly significantly smaller (0.05 µ2) than those of other cells studied. These studies provided added proof that monocytes probably are not present in the thoracic effluent of the calf.
4. Nuclear bodies were found to be present only in lymphocytes. They were present on the average in 12 per cent of thin sections of cells in this class. In contrast to nuclear bodies of other nonblood cells,20 in lymphocytes they were not associated with the nucleolus, were smaller in overall diameter, and often contained practically no electron opaque central portion.