Heisenberg reformulates his views on the group properties of the Hilbert-space I.

Pauli replies to the letter of Lee and Yang of 12 February. He talks about My-decay and B-particles, and about recent experiments on parity non-conservation.

Heisenberg talks about a calculation which leads to a result different from that given by Pauli in his last letter.

Heisenberg continues his mathematical argument of his last letter.

Pauli asks why in Heisenberg's calculations the force-potentials are always real.

Pauli makes further comments on Salam's paper in the light of Fierz's comments on it. He writes on Delbrück's paper about the relation between physics and biology and makes some personal remarks on his "mirror-complex" and about Salam.