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.

Pauli sends forms to be filled out for CERN.

Pauli encloses a letter by Heisenberg and asks Källén to write to him.

Pauli expresses his doubts whether Källén's mathematics has anything to do with physics. He talks about Heisenberg's approach to the problem of field theory.

Pauli talks about the progress made by Taylor in Princeton and Bogoljubov in dispersion relations and he writes about the article for the Springer Handbuch.

Pauli talks more about Springer and time-dependent wave-packets in a Lee model with complex roots. He writes on the interpretation of isotopic spin.