Källén replies to Pauli's letter of the 2 February 1958 on the product of two or more field operators.

Källén talks about some calculations in connection with Lee's model.

Källén discusses eigenvalues of a Hamiltonian in his particle theory.

Källén writes on the prospect of publishing a paper on the Lee model together with Pauli and on some mathematical difficulties in connection with this model.

Källén sends a draft of their common paper with some remarks about the calculations.

Källén sends some details of his work (together with Pauli) on Lee's model of a renormalizable field theory.

Källén gives details of the mathematical technique of his work on quantum electrodynamics.

Källén discusses aspects of his theory on mass spectrum, and also the properties of functions. He makes some personal remarks about Jouvet.

Källén replies to Pauli's second question in his letter of 23 January.

Källén thanks Pauli for an offprint of a paper by Pauli that is helpful for his calculations on the Lee model. He writes on the question of the unitary of the S-matrix in this model.