The B cell activating factor of the TNF family (BAFF, also known as BLyS, TALL-1, THANK, and zTNF4) is an important survival factor for B lymphocytes. Our previous study has demonstrated that the final purified material of human soluble BAFF (refolded hsBAFF) synthesized in Escherichia coli is biologically active in a validated induced human B lymphocyte proliferation bioassay. In this study, the administration of refolded hsBAFF to isolated mouse B lymphocytes and mice was carried out to study the immunological effects of hsBAFF on in vitro and in vivo B lymphocytes. The results showed that splenic B lymphocyte proliferation significantly increased after hsBAFF administration (in vitro 1, 2, 3, 5 microg/ml and in vivo 0.01, 0.5, 1.0 mg/kg body mass). An oppositely elevated immune response of B lymphocyte to LPS stimulation after hsBAFF administration (1, 2.5, 5 microg/ml) and a significantly elevated change after treatment with hsBAFF and costimulation with anti-IgM (2.5 microg/ml) was observed in vitro, respectively. A similar change existed also in hsBAFF-treated mice on the 8th postexperiment day, but the value with anti-IgM alone didn't increase compared to normal control in vitro. We found that the treatment of mice with hsBAFF resulted in a developmental maturation of T1 B lymphocytes to T2 and mature B lymphocytes by detecting distributions of splenic CD21(lo) with CD45R/B220(+) and CD21(hi) with CD45R/B220(+) subsets. These results suggest that the refolded hsBAFF synthesized in Escherichia coli may enhance immune responses in the body by regulating the proliferation, differentiation, and immune response of B lymphocytes.