Monocytes are the largest of the circulating white blood cells. Their cytoplasm stains a grayish blue and has a “ground glass” appearance due to numerous fine purplish granules. They often have vacuoles. The nucleus is often irregularly shaped with folds or indents and lacy chromatin.

Activated lymphocytes can be mistaken for monocytes due to their larger nature and more abundant cytoplasm. However, activated lymphocytes have a clear blue cytoplasm with a dark blue periphery instead of the ground glass cytoplasm. The nucleus is also more clumpy instead of lacy.