We established two different IL-5-dependent Ly1⁺ early B cell lines in long-term bone marrow culture system. One of them (J-87) is stromal cell (ST2) dependent and the other (T-88) is ST2 independent. Both J-87 and T-88 are B220⁺, Ly1⁺, slgM^-, la^-, Thy1^-, and IL-2R⁺, and respond to IL-3 and IL-5 in the presence of ST2. The T-88 can proliferate only in response to IL-5 in the absence of ST2. Southern blot analysis using J_H probe revealed that configuration of IgH gene of both cell lines shows rearranged pattern. Binding assay for radiolabeled IL-5 to T-88 demonstrated that T-88 has two classes of IL-5 binding sites (low and high affinity) on the membrane. These data strongly suggest that there are IL-5-sensitive stages at both stromal cell-dependent and stromal cell-independent phases in early B cell development.