In 1955, he published one of the first monographs on quantum electrodynamics, Kovarijantna kvantna elektrodinamika (in Croatian). With French physicists Jacques Bros and Henri Epstein he worked on setting up analyticity properties required for the use of dispersion relations in high energy collisions. Epstein, Glaser and Arthur Jaffe proved that (Wightman) quantum fields can necessarily have negative energy density values. Together with Henri Epstein, he found a new approach to renormalization theory called causal perturbation theory, where ultraviolet divergences are avoided in the calculation of Feynman diagrams by using mathematically well-defined quantities only.