Letter from Howard Walter Florey to C H C Toussaint, Willesden Chest Clinic, Pound Lane, London Has enquired about use of streptomycin without PAS - says this is done at Oxford - has been told that only effect which combination of PAS and streptomycin seems to have, is to lessen incidence of appearance of forms resistant to concentrations of streptomycin therapeutically obtainable. 12 January 1951