leukosit nongranular.

Merriam-Webster Online Dictionary
agranulocyte (noun)
a white blood cell without cytoplasmic granules
Agranulocyte (Wikipedia)
Not to be confused with agranulocytosis.
Monocyte under a light microscope (40x) from a peripheral blood smear surrounded by red blood cells.

Agranulocytes, also known as mononuclear leukocytes, are white blood cells with a one-lobed nucleus. They are characterized by the absence of granules in their cytoplasm, which distinguishes them from granulocytes. Normal hematologic blood values of MLs are about 35% of all white blood cells. The 2 types of agranulocytes in the blood circulation are lymphocytes and monocytes.[citation needed] A third type of agranulocyte, the macrophage, is formed in the tissue when monocytes leave the circulation and differentiate into macrophages.[citation needed]

Lymphocytes are much more common in the lymphatic system, and include natural killer T-cells. The blood has three types of lymphocytes: B cells, T cells and natural killer cells (NK cells). B cells make antibodies that bind to pathogens to enable their destruction. CD4+ (helper) T cells co-ordinate the immune response (they are what becomes defective in an HIV infection). CD8+ (cytotoxic) T cells and natural killer cells are able to kill cells of the body that are infected by a virus. T cells are crucial to the immune response because they possess a unique 'memory' system which allows them to remember past invaders and prevent disease when a similar invader is encountered again.

Reference ranges for blood tests of white blood cells, comparing mononuclear leukocytes amount (shown in yellow) with other cells.

Monocytes share the "vacuum cleaner" (phagocytosis) function of neutrophils, but are much longer lived as they have an additional role: they present pieces of pathogens to T cells so that the pathogens may be recognized again and killed, or so that an antibody response may be mounted. Monocytes are also known as macrophages after they migrate from the bloodstream and enter tissue.

Other white blood cells which are not agranulocytes are mainly the granulocytes: neutrophils, eosinophils and basophils.