Sendai and measles viruses were tested for their interferon-induced capacity in human lymphoblast cells. Sendai virus reproducibly induced considerable amounts of interferon (I research reference interferon unit/10(3) cells), but no increase in infectious virus titre was observed. Two Edmonston-Enders strains of measles virus grew very well. The attenuated (A) strain was a good interferon inducer (4 units/10(3) cells), while the virulent (V) strain induced only minimal amounts at a high multiplicity of infection. Pre-treatment of the cells with 5-iododeoxyuridine (IdUrd) had no effect on the growth of Sendai or EE measles virus and on interferon yields induced by Sendai virus. It slightly potentiated interferon induction by EE measles virus. Partial u.v.-light-inactivation of Sendai virus infectivity resulted in a parallel loss in interferon-inducing capacity.