In respect to the dose of antibiotics and the period of administration, the author studied the influence of antibiotics on antibody production in mice infected with Rickettsia tsustugamushi. Chloramphenicol was used as the test antibiotic, and the antibody production was estimated by the Weil-Felix reaction and cellular figure of the ascites. The results were as follows: 1) When chloramphenicol was given to mice in the begginning period, 24 hours after inoculation of R. tsutsugamshi, the antisera of mice showed a positive agglutination reaction to 20× to 40× dilution. 2) When chloramphenicol was given to mice in the later period, 8 days after inoculation of R. tsutsugamushi, the antisera of mice showed a positive agglutination reaction to 80× to 120× dilution; a almost similar result in the control group of mice not given chloramphenicol, 3) The agglutination titer of antisera was a little higher in the mice continuously administered small doses of chloramphenicol than in those administered once a big dose. 4) In the group of mice given chloramphenicol in the begginning period, the cellular examination of the ascites revealed the increase of histiocytes and monocytes, and the slight decrese of serous cells and lymphocytes. In those given chloramphenicol in the later period, the increase of serous cells, lymphocytes as well as histiocytes was observed; nearly the similar cellular figure to those in mice receiving no chloramphenicol administration. From these results, it is considered that the administration of chloramphenicol has a very great influence on the production of antibody.