Mammalian Erythrocytes.

http://en.wikipedia.org/wiki/Red_blood_cell

Mammalian erythrocytes are unique among the vertebrates as they are non-nucleated cells in their mature form.

  These cells have nuclei during early phases of erythropoiesis, but extrude them during development as they mature in order to provide more space for hemoglobin. 

In mammals, erythrocytes also lose all other cellula rorganelles such as their mitochondria, Golgi apparatus and endoplasmic reticulum.

As a result of not containing mitochondria, these cells use none of the oxygen they transport; instead they produce the energy carrier ATP by the glycolysis of glucose and lactic acid fermentation on the resulting pyruvate.