30th July, 1947.
Dear Dr. Heidelberger,
We have been trying to arrange that you should stop in London from August 25th - 26th or 27th on your way back to America, but Pan American Airways state that any such change of plan must be made with their Prague office and that they have not the power or authority to do this in London. It seems, therefore, that, if you feel disposed to stay in London, it would be necessary for you to get in touch with the Prague office of Pan American Airways.
Perhaps it might be well if I gave you an indication of the point which I want to discuss with you, as it may be that your views are so clear cut that it would not be worth while your calling here. The Medical Research Council have recently taken over a large modern laboratory which used to be known as the London County Council Serum Institute, Carshalton, and which was run by the London County Council.  During the war, it was taken over by Burroughs Wellcome who produced enormous quantities of biological products throughout this period. With the large changes in hospital and other medical services in this country, the Medical Research Council have decided to develop the Institute as a centre for research in immunology. It is hoped to cover the whole subject of immunology both from the antibody and also from the antigen side.
Clearly, directing a project of this kind would involve extensive knowledge of the chemistry and physics of biological material, and it seemed to me that you were the man to run such a show. It would not be possible for me to make you a definite offer of the post now, because a matter like this would have to be considered by the Medical Research Council themselves, but I wanted to know really how you would react to the suggestion. We would regard a post of this kind as one of very high standing, involving the appointment of a good many assistants, medical, biological, chemical and physical and it may be that you would like the possibility of running a large department along these lines or at least of looking into the project more closely before rejecting the idea. I can promise you that you would get a very good reception from your scientific colleagues in this country, and I am sure you would feel at home among them. Probably you would be so kind as to give me some idea as to how your mind reacts to this plan. If you came here I could take you down to the laboratory, which is about 15 miles out of London and situated in extensive grounds of about 60 acres.
If you decide to make a short stop in London, would you kindly let me know not only the day of your arrival but the number of the flight in which you are booked, so that we can find out the exact time of your arrival at the airport?
Yours sincerely,
E. Mellanby
P.S. The postal address of Pan American Airways office in Prague is: U.S. Lines, Pricopy 24, Prague.  The cablegraphic address is: Seapostes, Prague.