Hello Patterson,

I think you can check the biological potency of old progesterone by using ELISA assay .I mean you could use your progesterone as a antigen (make known concentration of standards) to check their immunological activity and specificity with the antibody!!!

How is an ELISA supposed to give an indication of biological potency?

If you are going to experiment with many animals, buy a new bottle.

If it's a small experiment and you have a super limited budget, then try with the dose you were indicated, maybe a bit higher if you want to compensate for some degradation over time.

The experiment is a trial, that is to say that I don’t want to invest time or resources in replacing the progesterone unless it is worth it. In Argentina, this is a significant limitation. The point is, I don’t want to get a false negative.

If your dose is X, sacrifice a rat with 10X or something like that? then if X didn't work but 5X did damage, you know that the progesterone you have is still good but you have to increase X maybe to 2X...

LD50 would be a possibility, if a little extreme, but I don’t know whether the storage breakdown products would be less, more or equally toxic. Thanks for the suggestion, though.

You could consider running some of your progesterone sample with an HPLC, or if you have access, MS.  This will tell you how pure and non-degraded the steroid is, and whether it is worth using.  

On the other hand, if its worth doing the experiment, then I agree with the answer above that it might be worth buying new progesterone - I understand your reasoning to use what you have, but as you acknowledge, a false negative could be even more costly in the long run.

These methods (HPLC and or MS) will tell you the state of your progesterone with accuracy more than the other tests mentioned, and will be faster and less effort, plus they will save animal use.

i like the "save animal use" option.