Thirty-nine samples of commercial products containing royal jelly were analyzed by liquid chromatography for their trans-10-hydroxy-2-decenoic acid (10-HDA) content. Most of the samples contained 10-HDA. Samples claimed to be pure royal jelly contained 1.98 to 6.37% 10-HDA (w/w). The 10-HDA content of samples claimed to contain royal jelly as an ingredient ranged from nondetectable to 1.28% (w/w).