In previous studies, we observed that thymulin 5CH could modulate the chronic inflammation response to BCG in an experimental infection, by increasing peritoneal B1 stem cells differentiation into phagocytes and improving bacilli phagocytosis efficiency into the infection site. Herein, the same protocol was used to study the effects of thymulin 5CH in a protozoan experimental infection. Male Balb/c mice were orally treated with thymulin 5CH or vehicle during 60 days after the subcutaneous inoculation of 2 x 105 units of Leishmania (L) amazonensis into the footpad. Then, washing inflammatory cell suspension from peritoneal cavity and spleen were harvested to be identified and quantified by flow cytometry and the tissue of infection site, as well as the local lymph node were harvested for histological examination and quantification. Treated mice presented increase in B1 stem cells percentage in peritoneal washing fluid and in spleen (p=0.0001), in relation to other cell types, and more organized and exuberant inflammation response in the infection site, with decrease in the number of parasites per field (p=0.05). No difference was seen in local lymph node histology. The results show that thymulin 5CH is able to improve B1 stem cell activation and Leishmania (L) amazonensis phagocytosis efficiency in mice, similarly to that observed previously in BCG experimental infection.