Human lymphocytes from peripheral blood (MNC) were separated on magnetic beads for the presence of different surface markers. Cells from positive and negative fractions were successfully immortalized by electrofusion with the heteromyeloma line CB-Fu2. B cells, which were separated on anti-CD 19 coated beads, could be immortalized at a rate between 10(-5) and 10(-4) even if the fusion was conducted with just a few hundred thousand cells. Comparison of the frequency of Ig-positive hybridomas in B cell, T cell, and unseparated MNC fusions indicated that also non-B cells may give rise to HAT resistant hybridoma clones, although the fusion frequency was low.