Swerilactones L-O (1-4), four unusual secoiridoids with unprecedented C₁₂ and C₁₃ skeletons, were isolated from the traditional Chinese herb Swertia mileensis. Compounds 1 and 2 had moderate inhibitory activities against the secretion of hepatitis B virus surface antigen (IC₅₀ = 1.47 and 1.20 mM, with SI < 1 and 1.53, respectively) and hepatitis B virus e antigen (IC₅₀ = 0.88 and >2.69 mM, with SI 1.62 and <1, respectively) in an antihepatitis B virus assay on the Hep G 2.2.15 cell line in vitro.