Pauli replies to Källén's solution in his letter of the 9 January 1958 with additional remarks about $\lambda_{o}$-particles, the identity of "isospace" with Hilbert-space and the fourfold degeneracy of the vacuum and its removal.

Pauli asks for Källén's opinion about Hurst's paper and quotes Thirring's comment.

Pauli comments on Källén's paper. There are differences between his part I and part II. He talks about states with one electron-positron pair and about Dyson's view that only renormalized interactions existing in nature.

Pauli talks about the excited state of a V-particle. The renormalization procedure seems artificial after the discovery that there exists more than one eigenvalue W$_{o}.

Pauli has some more considerations about the Lee model. He talks about some mathematical developments and their possible use for more complex systems with renormalization mathematics, especially quantum electrodynamics.

Pauli agrees with Källén about the correctness of the mathematics of the Lee-model. He writes on a question where to publish the paper. He agrees with Källén that Heisenberg's idea about the derivation of the electromagnetic field from spinor-fields must be wrong. He hopes that an improved renormalization mathematics might lead to a sort of eigenvalue-problem for the fine-structure constant.

Pauli writes on the criticism of a calculation by Mr Kurepa.

Pauli questions if there are other models beside Lee's model for renormalization. He describes a model by Thirring which avoids negative probabilities and which allows a transition to the limit of relativistic mesons.