Human B cell‐activating factor (BAFF) induces mouse surface IgM⁺ B cells of the immature type from bone marrow and of the immature types 1 and 2 from spleen, as well as of the mature type from spleen to increased longevity in tissue culture. BAFF does so polyclonally and without inducing proliferation in any of these B cell subpopulations. BAFF induces phenotypic and functional maturation of immature to mature B cells so that all immature cells loose C1qRp (AA4.1, 493) expression and type 1 immature cells up‐regulate IgD, CD21 and CD23. Immature B cells of types 1 and 2, upon pre‐incubation with BAFF, change their reactiveness to Ig‐specific antibodies so that they no longer enter apoptosis but now proliferate. However, BAFF does not seem to overcome negative selection of developing immature B cells in vitro.