At times it can be very difficult to differentiate monocytes from large lymphocytes. Monocytes may be mistaken for large lymphs when their cytoplasm stains too lightly, when the characteristic granules are indistinct, or when the nucleus is rounded or only slightly indented. Sometimes a cell will have the nucleus of a lymphocyte and the cytoplasm of a monocyte, or some other confusing combination of characteristics.
In order to properly identify the cell, it is necessary to weigh all of the characteristics together to determine which cell type it most resembles. In this microscopic field, the mononuclear cell on the left is a large lymphocyte and the mononuclear cell on the right is a monocyte.