Although mice are commonly used for studies of atherosclerosis, investigators have had no convenient way to quantify apolipoprotein (apo) B, the major protein of atherogenic lipoproteins, in this model. We now report an automated immunoturbidimetric assay for mouse apo B with an NCCLS imprecision study CV <5%. Added hemoglobin up to 50 g/L did not interfere with the assay, nor did one freeze–thaw cycle of serum samples. Assay linearity extends to apo B concentrations of 325 mg/L. We have used the assay to determine serum apo B concentrations under several atherogenic conditions, including the apo E “knock-out” genotype and treatment with a high-cholesterol diet. Our assay can be used to survey inbred mouse strains for variants in apo B concentration or regulation. Moreover, the mouse can now be used as a convenient small-animal model to screen compounds that may lower apo B concentrations.