Abstract
The administration of adrenocorticotrophic hormone and of cortisone was found to result in a reduction in the concentration of antipneumococcal antibody in the circulation of rabbits. This reduction occurred both when the hormones were administered at the beginning of immunization and after immunization was well advanced. Marked atrophic changes in lymphoid tissue and a diminution in the number of various types of mononuclear cells followed upon the hormone administration. The possible bearing of these observations on theories concerning the sites of antibody production is discussed.