B cell chronic lymphocytic leukemia (B-CLL) consists of the accumulation of malignant cells that apparently escape normal apoptotic regulation. We have studied the role of α4β1 integrin/fibronectin interaction in preventing apoptosis of these cells in vitro. B cells from 16 patients showed constant expression of α4β1 and little or no α5β1 B-CLL cells cultured on fibronectin or two previously described fibronectin recombinant fragments (H89 and H0) which contain the ligands for α4β1, consistently showed higher viability than control cells cultured on poly-lysine. The H89 fragment, containing the high affinity ligand CS-1, was the most efficient substrate with mean cell viability values of 72, 60 and 35% at days 2, 5 and 8 of culture, respectively. For control cells these values were 40, 27 and 15%, respectively. Parallel cell cycle analysis confirmed these results. The anti-apoptotic effect required direct contact with immobilized substrata since it was not observed when using B-CLL conditioned media alone or when clustering α4β1 with specific mAbs in suspension. Quantitation of the apoptosis regulatory proteins Bcl-2 and Bax revealed that cells cultured on the H89 fragment showed high/moderate levels of Bcl-2 (with some interpatient variation) and low levels of Bax resulting in an elevated Bcl-2/Bax ratio. These results indicate that adhesion of B-CLL cells to fibronectin upregulate the Bcl-2/Bax ratio and this may contribute to the anti-apoptotic effect induced via α4β1 integrin.