AL 8810 is an 11β-fluoro analog of PGF2α which acts as a potent and selective antagonist at the FP receptor. AL 8810 has weak intrinsic agonist activity on FP receptor preparations in the 200-300 nM range, yet it fully antagonizes the activity of the potent FP receptor agonist fluprostenol at this concentration, with EC50 values of approximately 430 nM. AL 8810 fully antagonized the bimatoprost-induced calcium mobilization in Swiss 3T3 fibroblasts at 100 µM, indicating that bimatoprost acts as an FP agonist in this preparation. The Ki for the inhibition of several potent agonists at the cloned human ciliary body FP receptor is in the range of 1-2 µM.