Human bone marrow (BM) is a relevant site for immunoglobulin (Ig) generation in vivo. The occurrence of BM cells capable of spontaneous and high rate Ig secretion for 14 d in vitro has been described previously. Accordingly, these cells provide a suitable model for studying terminal B cell maturation within the BM. We have reported recently that these BM cells are not totally differentiated when isolated from the body, as they require inductive signals from adherent stromal BM cells to complete their maturation. Interleukin (IL)-6 produced by these adherent BM cells was identified as one such signal. The present work shows that IL-6 was necessary, but not sufficient, for the induction of BM Ig-secreting cells, since the cytokine was unable to restore missing IgG in nonadherent BM cell cultures. Supernatants (SN) obtained from cultures of stromal adherent BM cells, either freshly isolated or derived from long-term BM culture (LTBMC), restored Ig secretion by nonadherent BM cells, suggesting that additional soluble factors from BM stromal cells were required. Fibronectin (FN) was identified as that factor, as can be deduced from the following findings: (a) stromal, but not nonadherent, BM cells constitutively produced FN; (b) anti-FN antibodies markedly reduced the IgG secretion in cultures of BM mononuclear cells (BMMC), and blocked the inductive effect of stromal cell SN on nonadherent BM cells, and such a blockade could be reversed by exogenous FN; and (c) finally, although neither IL-6 nor FN alone exerted any effect, the combination of both factors induced optimal Ig secretion by nonadherent BM cells. Furthermore, VLA-4 molecules seemed to be the FN receptor that was active in this culture system, as indicated by: (a) BM Ig-secreting cells exhibited the phenotype VLA-4+ VLA-5-; (b) mAbs directed to VLA-4 (anti-CD29 and anti-CD49d), but not those directed to other adhesion molecules, inhibited Ig secretion by BMMC cultures, and this effect was reversed by FN; (c) the inductive role of the entire FN molecule could be replaced by a fragment containing the CS-1 region, but not by a fragment containing the RGDS sequence; and (d) only mAbs anti-CD49d capable of blocking VLA-4-FN interaction inhibited induction by either the FN or the CS-1-containing fragment of FN.(ABSTRACT TRUNCATED AT 400 WORDS)