Letters to the Editor

Forgive my delay in commenting on Dr. C. C. McLean's earlier inquiry regarding the possible dangers arising from the addition of penicillin to the Salk vaccine. I have only recently received a copy of the revised "Minimum Requirements: Poliomyelitis Vaccine," issued by the Public Health Service of the U. S. Department of Health, Education and Welfare. In the first revision, dated April 12, 1955, it is stated (sect. 2.2) that the "virus for preparing vaccine is grown in tissue cultures containing . . . (c) penicillin and streptomycin in a concentration of not more than 200 units of micrograms per ml., respectively."