Several hybridomas supernatants capable of interferon beta (IFN-beta) or "IFN epsilon" ("IFN-eps") neutralizing or enhancing activities were obtained after in vitro immunization of BALB/c and C57 mice spleen cells and their fusion with Sp2/0 plasmacytoma cells. Besides rather low anti-IFN-beta or "eps" antibody secretion several cloned hybridoma fluids contained a factor potentiating anti-viral activity of the both IFNs. It is speculated that this activity is due to production by some hybridomas of another lymphokine.