Monoclonal antibody-secreting cell lines were isolated after transformation of peripheral blood leukocytes with Epstein-Barr virus. Blood samples were obtained from human donors having circulating antibodies against hepatitis viruses (HAB, HBV), rubella, or rabies virus and from a chimpanzee infected with HAV. Dextran-isolated leukocytes were submitted to Epstein-Barr virus infection at low cell concentrations (1 X 10(4) cells X ml-1). Proliferating clones could be observed in 50-100% of the cultures within 4-6 weeks. Out of 1 ml blood (1 X 10(6) leukocytes) 1-10 stable clones were isolated, secreting specific anti-viral antibodies. These clones were fused with an aminopterin-sensitive, ouabain-resistant, non-immunoglobulin producing mouse-human hybridoma (Org MHH.1). From such fusions 10-90% of the cultures yielded viable hybridomas of which 45% produced antibodies with the same specificity as of the parental EBV transformant. Immunoglobulin production of both EBV transformants and hybridomas was shown to be stable for more than 6 months and at a concentration up to 100 micrograms X ml-1 X 48 h-1. Chimpanzee EBV-transformed lymphocytes proliferated excellently in vitro. Mouse-human hybridomas, however, could be more easily cultivated, cloned and scaled up than the parental EBV-transformed lymphocytes. In conclusion, stable, monoclonal antibody-secreting cell lines of either human or chimpanzee origin could be isolated with an efficiency that exceeds by 10-100-fold standard murine hybridoma technology.