Pauli asks about Thirring's abilities.

Pauli regards Heisenberg's letter of 31 January as showing new progress. He talks about the mathematical difficulties of the Lee model and a general charaterization of the class of models with indefinite metric.

Pauli talks about the isotopic spin problem for charged particles.

Pauli is very optimistic about the results so far. He suggests as a title for the the joint paper: "On the interpretation of isotopic spin". He gives some moreconclusions. He makes some mathematical comments on unitary and pseudo-unitary transformations, and the dichotomy of the vacuum.

Pauli says that he reached similar conclusions to Heisenberg.

Pauli replies to Heisenberg's letter of 27 February. He thinks that understanding between him and Heisenberg may be impossible, because he regards Heisenberg's calculations and interpretation as "wishful thinking". There is a discussion of the parity problem.

Pauli sends a circular letter concerning Heisenberg's paper on isospin. He explains why he withdrew his participation in Heisenberg's publication.

Pauli expresses great interest in Frauenfelder's experimental results. He formulates three independent questions concerning parity violation to which an experimental answer can be found in principle.

Pauli asks how Frauenfelder will test the exclusion principle for positrons.

Pauli is against the interpretation given by Princeton physicists to certain Beta-decay experiments and against the two-component theory.