Human endothelial culture supernatant (HECS) contains factor(s) which promote the growth and the stability of hybridoma cells, and in these respects is superior to the commonly used feeder cells. In particular: HECS increases the recovery of hybridomas after fusion as compared to that in the presence of feeder cells; HECS can substitute feeder cells supporting the growth of hybridomas at the single-cell level under limiting dilution conditions; HECS prolongs the stability of human-mouse hybridomas producing human immunoglobulins (Ig). The active principle which is present in HECS can be different from IL-6. Furthermore the effects of a polyamine derivative, carboxyethyl gamma-aminobutyric acid (CEGABA), were studied on the growth of hybridomas and Epstein-Barr Virus-transformed cell lines. CEGABA enhances the growth of hybridomas through all the steps of hybridoma technology and induces the formation of a large number of EBV-transformed lymphocytes, when added to the culture medium immediately after EBV infection. However, CEGABA shows only a moderate effect on the stability of EBV-transformed cell lines over time, and does not effect the growth of stabilized cell lines. It is possible that CEGABA acts on cells other than EBV-transformed lymphocytes (in fact, after EBV infection all types of mononuclear cells from the blood are present in the culture) and indirectly promotes the growth of EBV-transformed lymphocytes.