To analyze whether the phenotypic abnormalities observed in lymphotoxin- α(-/-) (LTα(-/-) mice are intrinsic to the hemolymphoid system itself or dependent on stromal elements, wild-type (WT) mice were reconstituted with bone marrow (BM) cells enriched for hemopoietic stem cells from LTα(-/-) animals. WT mice reconstituted with LTα(-/-) c-kit+Lin-Sca-1+ BM cells do not maintain follicular dendritic cell (FDC) networks and do not form primary follicles, while clear segregation of B and T cells could be observed. Furthermore, IgM+IgD- B cells, MOMA-1 (anti-metallophilic macrophages), ERTR-9 (anti-marginal zone macrophages), and MECA-367 (anti-MAdCAM-1) were all absent from the splenic marginal zone. Surprisingly, however, the expression of MOMA-1, ERTR-9, and MAdCAM-1 was normal in the lymph nodes of mice reconstituted with LTα(-/-) cells. In addition, peanut agglutinin- positive germinal centers were observed in both the spleen and mesenteric lymph nodes, although in the absence of detectable FDC. Furthermore, in animals reconstituted with a mixture of LTα(-/-) and WT c-kit+Lin-Sca- 1+, GC contained either predominantly LTα(-/-) B cells or WT B cells. These results suggest that although the formation of primary follicles, FDC networks, and the splenic marginal zone are all dependent on hemopoietically derived LTα, germinal center formation and the expression of MAdCAM-1, MOMA- 1, and ERTR-9 in lymph nodes are not. Our results also suggest that the disturbed B-T cell separation in LTα(-/-) mice is unrelated to defects in the marginal zone.