The aim of this study was to establish hybridomas capable of long-term production of human monoclonal antibodies (mAbs). Heterohybridization was performed between the mouse myeloma cell line P3X63Ag8.653 and activated human peripheral blood lymphocytes (PBL). In order to achieve better retention of human chromosomes, as well as to improve the stability of the heterohybrids, one HAT-sensitive immunoglobulin (Ig)-non-secreting human x mouse (h x m) heteromyeloma was fused for a second time with activated human PBL. In this way, a panel of HAT-sensitive Ig-non-secreting h x h x m heteromyelomas was obtained and tested for its ability to generate stable human Ig-secreting heterohybrids with activated human PBL. Six lines were selected on the basis of their enhanced characteristics of fusion efficiency and genetic stability. When fused with in vitro immunized human PBL, they generated several h x h x h x m hybridomas stably secreting high yields (10-23 micrograms/ml/24 h) of human mAbs reactive with recombinant HBV core antigen (rHBcAg). Moreover, a continuous production of human Ig was observed when two h x h x m heteromyelomas, previously made ouabaine-resistant, were hybridized with EBV-transformed lymphoblastoid cell lines. These h x h x m heteromyelomas are ideal fusion partners for the production of human mAbs.