He is most well known for his work on the Gell-Mann–Nishijima formula, and the concept of strangeness, which he called the "eta-charge" or "η-charge", after the eta meson (η).

This formula was pivotal for the later development of the quark model by Gell-Mann[5] and George Zweig[6][7] in 1964 (independently of each other).

[3] In 1958, he moved to the United States and joined the Institute for Advanced Study in Princeton, New Jersey.

[3] In 1966, he returned to the University of Tokyo, where he founded a theoretical physics research group and served in some administrative positions.

His last subjects of research were color confinement and noncommutative quantum field theory.