Sterol O-acyltransferase (SOAT, also known as acyl-CoA:cholesterol acyltransferase (ACAT), EC 2.4.1.26), an endoplasmic reticulum (ER) membrane protein, catalyzes the synthesis of cholesteryl ester (CE) from free cholesterol and long-chain fatty acyl-CoA. SOAT has been postulated as a target for modulation by a new type of antiatherosclerotic agent. Recent molecular biological studies revealed the existence in mammals of two different SOAT isozymes, SOAT1 and SOAT2.1–4 SOAT1-selective inhibition may cause detrimental effects,5–7 whereas SOAT2-selective inhibition has consistently shown antiatherosclerotic activity.8,9 Therefore, it is important to determine the selectivity of inhibitors toward the two SOAT isozymes for development as new antiatherosclerosis agents. Our group has focused on the discovery of SOAT2-selective inhibitors of microbial origin in cell-based assay or an enzyme assay using microsomes prepared from SOAT2-expressing Chinese hamster ovary (CHO) cells. During the course of our screening program, two diketopiperazines, 1 and amauromine10 (2) (Figure 1), were isolated as SOAT2 inhibitors from the culture broth of actinomycete strain Nocardiopsis sp. KM2-16. Amauromine was originally isolated as a vasodilator from the culture broth of the fungus Amauroascus sp. Yin et al. 11 reported that 1 was produced by bioconversion from cyclo-L-tryptophan-L-tryptophan using two recombinant enzymes involved in acetylaszonalenin biosynthesis.12 Thus, 2 and structurally related acetylaszonalenin were fungal secondary metabolites. In this study, we showed that 1 and 2 were isolated as actinomycete secondary metabolites and that 2 selectively inhibited SOAT2 activity.