Effect of IL-10 on the OI. EBV-infected PBL from 11 seropositive and one seronegative (EBV S⁻) donors were seeded at various cell concentrations in flat-bottomed microtiter plates in medium control or medium containing 100 U/mL of IL-10. Four to 5 weeks later the transformation was visually scored. The results are expressed as OI index representing the minimum number of cells/well required to achieve 50% inhibition of EBV-induced transformation.