In hepatic mitochondria, the outer membrane enzyme, carnitine palmitoyltransferase-I (CPT-I), appears to colocalize with contact sites. We have prepared contact sites that are essentially devoid of noncontact site membranes. The contact site fraction has a high specific activity for CPT-I and contains a protein at 88 kDa that is recognized by antibodies directed at two different peptide epitopes on CPT-I. Similarly long-chain acyl-CoA synthetase (LCAS) specific activity is high in this fraction; a protein at 79 kDa is recognized by an antibody against LCAS. Although activity of carnitine palmitoyltransferase-II (CPT-II) is present, it is not enriched in the contact site fraction, and a protein of 68 kDa weakly reacted with anti-CPT-II antibody. Likewise, carnitine-acylcarnitine translocase (CACT) protein is present, but at a somewhat reduced level. Using an analytical continuous sucrose gradient, we demonstrate that the activities of CPT-I and LCAS and their associated immunoreactive proteins are present in a constant amount throughout the contact site subfractions. The enzymatic activity of CPT-II and its associated immunoreactive protein, as well as immunoreactive CACT, is absent in the lighter density gradient subfractions and is present in the higher density subfractions only in trace amounts. This heterogeneity of the contact site fraction is due to unvarying amounts of outer membrane and increasing amounts of attached inner membrane with increasing density of the subfractions.