Several million Americans have received two or more injections of Cox-type epidemic typhus vaccine during the past nine years (1). This is in accord with the policies of the Armed Forces and various civilian agencies of the United States which require all personnel being sent to potential zones of epidemic typhus to be immunized against this disease. Today, many of these previously immunized individuals are being sent overseas again either as members of the Armed Forces or as civilians participating in the programs of the State Department, the Economic Cooperation Administration, or other agencies. The question now arises as to whether these previously immunized individuals should receive the usual primary course of two injections of typhus vaccine or whether a single booster dose will suffice to recall and raise their typhus antibodies to the maximum obtainable level (2). Data bearing upon this problem are presented below.2