Rose hip has previously shown clinical efficacy in the treatment of osteoarthritis, and organic solvent extracts of rose hip have showed inhibition of cyclooxygenase-1 and -2. A petroleum ether extract of rose hip was fractioned by VLC on silica; on a C-18 column and by HPLC. Each step was COX-1/2 activity-guided. The bioassay-guided fractionation led to the isolation of linoleic acid (the IC50 for COX-1 was 85 microm and 0.6 microM for COX-2) and alpha-linolenic acid (the IC50 for COX-1 was 52 microM and 12 microM for COX-2). The COX-2/COX-1 ratio was 0.007 for linoleic acid and 0.2 for alpha-linolenic acid. Linoleic acid and alpha-linolenic acid contribute to the COX-1 and -2 inhibitory activity of rose hip.