Pauli talks about a confusion in Källén's calculations in the last letter.

Pauli has more thoughts about the Lee model. He asks Källén to work out the free-of-infinities formalism and gives an indication how to do it. He asks about a generalization of the method of Cauchy's integral-proposition for quantum electrodynamics.

Pauli writes on a question about the S-matrix in Lee's model. "Is it unitary?"

Pauli wants to write first to Lee before the proposed paper by Pauli and Källén is written. He writes on a definition of a "ghost state". He expresses doubts about the Tamm-Dancoff approximation in the case of more general systems. He makes some mathematical remarks.

Pauli thanks Källén for the manuscript. He writes about the Tamm-Dancoff method and some known negative attitude towards it. Small changes in the manuscript and the re-writing of the appendix.

Pauli has some more suggestions for minor changes in the paper.

Pauli replies to Källén's letter of the 10 March 1955. He suggests destroying the Lorentz invariance through the cut-off of the interaction energy in momentum-space. He writes on the validity of Born's approximation.

Pauli writes on a paper by Bosco and Fubini "on the mathematical structure of the charged scalar meson theory". He talks about the progress of the work by Lehmann and Jost and about the CERN meeting.

Pauli writes about a report from the USA by Jost that Bogoljubow has advanced more than anybody else in the mathematics of dispersion relations.