Mice homozygous for a targeted null mutation of lymphotoxin-α (LTα) are born without lymph nodes (LN) or Peyer's patches (PP) and with altered splenic architecture. To investigate the mechanism of failed LN organogenesis, we transferred bone marrow (BM) from Thy 1.2 LTα-deficient or Thy 1.2 wild type mice to lethally irradiated 8-12-week-old Thy 1.1 wild type recipients. Six to 10 weeks later, reconstitution of LN and spleen with Thy 1.2 cells was similar whether the BM was derived from LTα-deficient or wild type donors. In contrast, reconstitution of irradiated LTα-deficient mice with wild type BM did not induce the development of detectable LN, although reconstitution of the spleen occurred appropriately. The expression and regulation of the lymphocyte adhesion molecule L-selectin from the LTα-deficient mice appeared normal. These data indicate that LTα-dependent interactions must occur during development in order for LN genesis to take place; however, lymphocyte expression of LTα is not required for these cells to home to existing LN structures.