A monocyte is a type of leukocyte (white blood cell) that patrols the bloodstream in search of antigens. In time, monocytes migrate into the tissues and develop into macrophages.

Monocytes are comparatively large (12 to 20 microns in diameter) and, when stained, have abundant blue-gray or sometimes pale pink cytoplasm with small, generally indistinct, granules. Vacuoles are often present in the cytoplasm. The nucleus is irregular, frequently with delicate folds and often lobulated. The mature monocyte has no nucleoli. A milliliter of normal blood contains up to a million monocytes.