Cross-linking of B cell antigen receptor (sIg) elicits different biological responses, including cell activation, proliferation, differentiation, anergy and cell death depending on the maturational stage of the cell. We established the tumor cell lines HF-1.3.4 and HF-4-9 from two patients with follicular lymphoma. Both cell lines carry the characteristic t(14;18) chromosomal translocation and display constitutively overexpressed Bcl-2. HF-1.3.4 represents a mature B cell with sIgG and several somatic hypermutations in its Ig genes, while HF-4-9 is a less mature B cell, expressing sIgM and only a few mutations in its Ig genes. Cross-linking of sIg with antibodies leads to apoptosis in HF-1.3.4 cells but not in HF-4-9 cells. Triggering of sIg induced, within seconds, identical tyrosine phosphorylation of p53/56lyn protein tyrosine kinase (PTK) and p55blk PTK in both of the cell lines; however, a prominent tyrosine phosphorylation and activation of p72syk PTK only in HF-1.3.4 cells. We conclude that p72syk PTK is of importance in relaying apoptotic signalling upon sIg cross-linking in the HF-1.3.4 cell line. Given the mature phenotype of the HF-1.3.4 cell line it serves as a model for the late negative selection during B cell ontogeny. Moreover, our results question the current concept that a constitutive overexpression of BcI-2 confers resistance to sIg ligation-induced apoptosis in lymphoma cells.