Purified rat liver ATP citrate lyase is shown to bind to the microsomal fraction of rat liver. Under the same conditions the enzyme does not bind significantly to the mitochondrial fraction or to the outer membrane prepared from the mitochondrial fraction. The binding component of the microsomal fraction is further identified as the endoplasmic reticulum, and a protein component of the membrane is involved in binding. Binding decreases with increasing salt concentration. It requires more than 50 mM potassium phosphate or 60 mM potassium chloride to decrease binding significantly whereas complete inhibition of binding is observed in the presence of 0.01 mM CoA. The binding capacity of the microsomal fraction is shown to be high enough to allow most, if not all, the ATP citrate lyase present in rat liver to be bound to the microsomal fraction even when the enzyme has been induced over 10-fold by dietary manipulations. No significant difference has been found when comparing the binding of the phospho and dephospho forms of ATP citrate lyase.