The effects of Con A on homing patterns of mouse spleen cells were investigated. After 1 to 18 hr incubation with or without the mitogen, cells were labeled with 51Cr and injected i.v. into syngeneic hosts. Treatment of spleen cells with Con A decreased localization in lymph nodes 75 to 95% and inhibited homing to recipient spleens 17 to 50%. Conversely, localization in liver and lungs was increased. Migration of cells labeled with both 124I Con A and 51Cr revealed that cells binding the largest amount of Con A were trapped in lungs whereas cells migrating to lymph nodes and spleen had a much lower Con A content. Elution of Con A from lymphocyte surfaces with methyl α D mannoside (MAM) reduced the percentage of cells localizing in lungs, with a concomitant increase in the percentage of cells migrating to lymph nodes. However, even after MAM treatment, a cell population with a relatively large Con A content localized in the lungs. These data indicate that both surface bound Con A and Con A induced changes in lymphocyte membranes inhibit migration of Con A activated cells to recipient lymph nodes. They suggest that after Con A treatment lymph node seeking cells do not migrate normally because of preferential sequestration in lungs of lymphocytes with either Con A binding sites of increased density or avidity or with an increased propensity for Con A internalization.