The injection of yellow fever virus fixed for mice together with immune serum (1) is an effective method of immunization against yellow fever but cannot be applied widely because of the difficulty in securing adequate amounts of immune human serum. The substitution of hyperimmune sera of animal origin has been suggested by Pettit and Stefanopoulo (2) and the feasibility of that procedure has been studied by Theiler and Whitman (3). Simplification in yellow fever immunization has been accomplished by the procedure of Sellards and Laigret (4), later modified by Laigret (5) in which active but allegedly attenuated virus is used without serum. The safety of this process has been questioned by Theiler and Whitman (6) and its efficiency by Findlay (7).

Immunization against dog distemper (8) and Rift Valley fever (9) by the use of inactivated viruses has revived interest in the application of this method to yellow fever immunization, although the results of previous investigations were unfavorable (1, 2, 10, 11, 12, 13).

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