A complex of metabolites consisting of two isomeric cyclic acylpeptides was isolated from a culture of Bacillus sp. A1238 by successive chromatographies on Amberlite XAD-7, silica gel and silica ODS columns. By a combination of spectroscopic and chemical analyses, the two subcomponents were identified as isomers of halobacillin, and the complex was designated isohalobacillin. Each molecule of isohalobacillin subcomponents contains either a 3-hydroxy-l-oxo-13-methyltetradecyl or a 3-hydroxy-1-oxo-12-methyltetradecyl moiety in place of a 3-hydroxy-loxopentadecyl moiety that is found in the halobacillin molecule. In a cell-free assay, isohalobacillin inhibited acyl-CoA: cholesterol acyltransferase by 50% at a concentration of 50μM. When added to a culture of macrophage J774, the agent inhibited oxidized low density lipoprotein-induced synthesis of cholesteryl ester from [¹⁴C]oleate without affecting surface binding, internalization and degradation of the lipoprotein in the cells.