An enhanced memory response, as shown by increased titers of specific immunoglobulin A (IgA), was seen in intestinal secretions from isolated Thiry-Vella loops in rabbits primed orally with live, locally invasive Shigella sp. X16 and challenged 60 days later with a single oral dose of the same antigen. Heat-killed shigella preparations, when used as either the priming or challenge antigen, did not elicit such a memory response in this system. In the present study, the role of antigen form and dosage in eliciting the enhanced local IgA response was investigated. A noninvasive strain, Shigella flexneri 2457-0, was capable of significantly enhancing the mucosal IgA memory response, whereas heat-killed Shigella sp. X16 was unable to augment the local IgA response, even when the priming dose was increased 100-fold. A proposed mucosal adjuvant, DEAE-dextran, given orally with live Shigella sp. X16, did not enhance the local IgA response. Viable, noninvasive shigellae were effective priming agents in enhancing the local IgA memory response. The poor mucosal response to heat-killed shigella preparations is thought to be related to an ineffective delivery of nonviable bacterial antigens into gut-associated lymphoid tissues. The ability of the live, noninvasive strain to elicit a vigorous local IgA memory response when given orally to rabbits was consistent with previous findings that live preparations elicit the best mucosal IgA response.