In this study, we investigated in the BALB/c model, the dose-dependent protective potential of previous infection with Leishmania infantum parasites, against a high-dose challenge and showed for the first time that low-dose imprinting conferred substantial spleen resistance. Mice were immunized for 1 month or 5 months by IV route with parasite inocula ranging from 10(4) to 10(7) and from 10(3) to 10(5), respectively, and challenged for 1 month with 3 x 10(7) parasites. Liver protection was directly proportional to the parasite dose used for infection and reached 90-95% whereas, only low doses (< or =10(5)) protected spleen. Maximal spleen resistance (80%) was reached in mice infected for 5 months with 10(5) parasites. In most cases, protection was accompanied in spleen, by restored in vitro responses to Leishmania antigens. Analysis of anti L. infantum isotype responses and in vitro antigen-induced cytokine production, indicated that the acquired protection was irrespective of a Th1/Th2 imbalance.