Caper spurge (Euphorbia lathyris L.) seed oil contains a series of diterpenoids known as Euphorbia factors, or L-factors, L1–L9. They are esters of several polyols (lathyrol, epoxylathyrol, hydroxylathyrol and ingenol) and account for about 3–5% of the oil. The percentage of ingenol-based L-factors is very low, less than 5% of the diterpenoid fraction, but some of them (factors L5 and L6) are responsible for the irritant and co-carcinogenic activities of the oil. This paper reports an HPLC-UV and HPLC-positive-ESI-MS analysis of the diterpenoid fraction of caper spurge seed oil before and after selective hydrolysis of ingenol-based L-factors. Separation of lathyrane polyols and esters, and ingenol and its esters was achieved using a chromatographic system consisting of a C18 stationary phase and acetonitrile:water as mobile phase. A new macrocyclic constituent, the deoxy Euphorbia factor L1, was identified in the oil. Copyright © 2001 John Wiley & Sons, Ltd.