The $\textit{International Unit per Milliliter}$ is a unit for 'Serum Or Plasma Level' expressed as $IU/mL$. The magnitude of one $IU/L$ depends on the material, so there is no single conversion multiplier. Following the mass-equivalent treatment of $IU$ (see unit:IU), this is dimensioned as a mass concentration.
International Unit per Milliliter
International Unit per Millilitre