The role of serum antitoxic antibody in protection against the dysentery caused by Shigella dysenteriae type 1 (Shiga's bacillus) was studied in monkeys fed 10(10) virulent organisms after parenteral immunization with a formalin-inactivated Shiga toxoid preparation standardized in mice. Two 125-microgram doses of toxoid adsorbed on aluminum hydroxide adjuvant and given 14 days apart provided mice with a 54-fold increase in resistance to parenteral toxin. In rhesus monkeys (Macaca mulatta), the same regimen of toxoid permitted the safe parenteral administration of toxin in incremental doses ranging from 100 to 1,000 mouse 50% lethal doses and resulted in correspondingly high titers of antitoxin in serum. Nevertheless, the immunized monkeys responded to orally administered Shiga bacilli by development of diarrhea and dysentery that was as severe as (or more severe than) the response of unimmunized controls. The failure of extraordinarily high levels of circulating antibody to protect against experimental shigellosis suggests that the intestinal mucosal sites of toxinmediated response are beyond the reach of systemic antitoxin.