Antigen specific B cells (ASC) that circulate after oral immunization with the typhoid vaccine Ty21a display cell surface determinants which are potentially involved in B cell differentiation and homing to mucosal sites. These ASC were isolated from peripheral blood after oral Ty21a, and dual labelled for binding of typhoid antigen and expression of various cell surface determinants: alpha 4 integrin (CD49d), CD45RO, CD45RA, L-selectin, CD44 and CD11a. Of particular interest was the finding of CD45RO expression on ASC. A comparison of cell surface determinants on typhoid-specific cells was also made following binding to high endothelial venules on peripheral and mesenteric lymph nodes, and venules in the lamina propria of the small intestine. Generally more typhoid ASC bound to mesenteric compared with peripheral lymph node. More ASC expressing CD45RO and alpha 4 integrin (CD49d) were bound to mesenteric lymph node and small intestine than to peripheral lymph node. When expressed as a fraction of total ASC, the difference was statistically significant only for CD45RO binding to small intestine versus peripheral lymph node. No differences in expression of other homing markers on bound ASC were seen.