Pauli talks about questions in connection with the preparation of a lecture on problems of quantum electrodynamics: 1) about Lee's non-relativistic models for renormalization, 2) about longitudinal photons, 3) about a paper by Green published in: Physical Review 95, 548 - 1954.

Pauli thanks Källén for his reply. He objects to his views on the gauge-invariance. He writes on the danger for a theoretical physicist to stick to one special formalism. This is also Källén's mistake in his formulation of the Lee-model.

Pauli replies to Källen's letter and continues the topics in Pauli's last letter. The coupling-free case has to be eliminated from Lee's model because it does not exist in nature.

Pauli sends a list of people to whom preprints of their paper should be sent. He comments the reaction of Heisenberg and his pupils to the paper.

Pauli writes on the non-physical ("pathological") solutions of quantum electrodynamics in case of strict validity of Lorentz-invariance. The mathematical consequences of this: "ghosts" and macroscopic acausalities and the postulate that for large photon- and quantumnumbers of electrons the classical theory must result as limiting case of quantum electrodynamics. One has to look if the equations of quantum electrodynamics have solutions which fulfil these three consequences.

Pauli writes of his going to the conference in Pisa where he will discuss Källén's letter. He talks about Landau's papers whose general gist is like that of Källén's and about his doctoral student Enz and his work.

Pauli talks about offprints of their common paper.

Pauli asks him to send a copy of his work to Heisenberg. He mentions an error found in Heisenberg's paper.