Following i.v. injection, 2,4,6-trinitrophenol (TNP)-primed memory cells localized in recipient lymph nodes draining a footpad injection of TNP-hemocyanin (TNP-KLH) in greater numbers than in contralateral nodes draining a p-azobenzenearsonate-coupled KLH injection. Such hapten-specific, unilateral memory B cell localization was still observed in immunosuppressed mice when antigen injections were given as long as 4 days prior to the memory cell transfer. The memory cells could be challenged to form plaque-forming cells by footpad injections of TNP-labeled Brucella abortus at 5 days, but not one day, after cell transfer. The present studies further clarify some parameters of this adoptive memory, as a model for the study of persistent local memory. Measures that promoted the unilateral lymph node retention of 125I-labeled antigen also facilitated unilateral accumulation of TNP-specific memory cells. Such measures included pretreatment of the recipients with cyclophosphamide, rather than gamma irradiation, injection of anti-carrier antibody the day before antigen, or use of small doses of preformed immune complexes instead of antigen alone. In general, a high ratio of lymph node-to-spleen and lymph node-to-blood concentration of antigen in recipients appeared crucial for unilateral localization of memory B cells. Splenectomy of recipients prior to cell transfer enhanced the difference in plaque-forming cell responses between draining and contralateral nodes, but decreased their difference when performed 1 day after cell transfer, suggesting that the spleen may have served as a trap for memory cells. I.v. injection of antigen at the time of B cell transfer also interfered with unilateral localization. The results demonstrate that in the presence of persisting depots of antigen within lymph nodes (and absence of significant amounts of antigen elsewhere), memory B cells can be retained locally without activation into antibody-secreting cells. This mechanism may, therefore, by responsible for the phenomenon of local, humoral, immunological memory.