Hello, Alaleh.

After searching a few google sites, I come to know IU and U are in mixed use. But they are the same. Scientific expression is IU, and this is written as U probably due to the confusion between IU (international unit) and 1U (one unit) arising from practical clinics. Thus, people often use U to putting dosage number in front of U.

Dear Sang,

I concluded same as you but you confirmed what I understood about these two.

Thank you so much

I am not sure if it is applicable for your material, but when I asked a manufacturer of cytokines on difference between "U" and "IU", I got the following answer.

"The specific activity unit and international unit are two discrete values, which are determined separate ways."

Kuniko Saeki Thank you for your response. I use IL-2 for T cell activation. However, I didn't get the answer about how to convert them. It is a confusing matter. Someones believe they are the same, while some believe they are not. I read in a website that even IU differs for the same products produced by different companies! So, I used a product with the same unit to my protocol!

International Units per mL or (I.U/mL) is the same as (U/mL) which their unit is (μmol /min)

The enzyme unit (U), or international unit for enzyme (IU) is a unit of enzyme's catalytic activity. 1 U (μmol/min) is defined as the amount of the enzyme that catalyzes the conversion of 1 micromole of substrate per minute under the specified conditions of the assay method.