The combination of mutant EL-4 thymoma cells and Epstein-Barr virus (EBV)-infection in the activation of human B lymphocytes was studied with two approaches: a) limiting numbers of B cells were first activated by EL-4 contact in order to expand the B cell population and then infected with EBV. Results show that EBV could induce further Ig synthesis, although was unable to determine proliferation or to generate immortalized lines from EL-4 activated cells. b) EL-4 cells were compared to conventional PBM as feeders for cultures of established lymphoblastoid cell lines (LCL) at low cell densities. EL-4 feeder activity was strictly dependent on the presence of phorbol-myristate-acetate (PMA) and supernatant from stimulated T-lymphocytes (T-SN). EL-4 feeders induced earlier proliferation peak and greater Ig synthesis by LCL. The latter effect could be also mediated by the addition of PMA and T-SN, even if a cooperating cell-to-cell signal by PMA and T-SN-sensitized-EL-4 cells could not be excluded. Altogether results indicate that EL-4 cells do not represent a clear advantage over classic PBM as feeders for cultures of established LCL at low cell densities.