Ernest Armstrong McCulloch OC OOnt FRS FRSC[1] (27 April 1926[2] – 20 January 2011)[3] was a University of Toronto cellular biologist, best known for demonstrating – in a partnership with James Till – the existence of stem cells.

In the early 1960s, McCulloch, and Till started a series of experiments that involved injecting bone marrow cells into irradiated mice.

In later work, Till & McCulloch were joined by graduate student Andy Becker, and demonstrated that each nodule did indeed arise from a single cell.

[citation needed] McCulloch's later research was on cellular and molecular mechanisms affecting the growth of malignant blast stem cells obtained from the blood of patients with Acute Myeloblastic Leukemia.

In 1969, McCulloch won the Canada Gairdner International Award with James E. Till in recognition of their development of the spleen colony technique for measuring the capacity of primitive normal and neoplastic cells to multiply and differentiate in the body.