He became Doctor of Medicine in 1892 and began his work at the Pasteur Institute in Paris in 1894, where, in the laboratory of Elie Metchnikoff, he described phagocytosis of bacteria by white blood cells. In 1898 he described hemolysis evoked by exposure of blood serum to foreign blood cells.
In 1900, he left Paris to found the Pasteur Institute in Brussels, and made his discovery that the bacteriolytic effect of acquired specific antibody is significantly enhanced in vivo by the presence of innate serum components which he termed alexine (but which are now known as complement). This mechanism became the basis for complement-fixation testing methods that enabled the development of serological tests for syphilis (specifically, the development of the Wassermann test by August von Wassermann). The same technique is used today in serologic testing for countless other diseases.
He became Professor of Bacteriology at the University of Brussels in 1907.