Over the next several years, he taught high school in Cornwall, Ontario and continued scientific work under Wright's direction.

[2] In his first years as physiology chair at the University of Toronto, Macallum and several other biologists trained by Wright (anatomy chair James McMurrich and pathology professor J. J. Mackenzie) fought to replace the Toronto medical school's traditional medical education with a curriculum based on biological science.

[5][6] Most of Macallum's scientific work involved measurements of small concentrations of salts and ions in biological fluids.

[1] Building on his ion measurement work, Macallum's subsequent long-term research focused on the ionic content of blood and other biological fluids.

He found that in many animals, ions of sodium, potassium, calcium and magnesium occur in close proportion to the ionic content of seawater, providing a strong argument for the marine origins of land vertebrates.