We describe a novel, immunospecific scintillation proximity assay for determining cholesteryl ester transfer protein (CETP) activity in total human serum and in reconstituted experimental mixtures. The assay is based on the measurement of radiolabeled cholesteryl esters transferred from a tracer dose of biosynthetically labeled high-density lipoprotein subfraction 3 to unlabeled apolipoprotein (apo) B-containing lipoproteins. The radioactivity content of the apo B-containing lipoprotein fraction can be evaluated without separating the donor from the acceptor lipoprotein substrates, and is measured through the formation of ternary complexes involving the radiolabeled apo B-containing lipoproteins, specific anti-apo B antibodies from sheep, and anti-sheep antibody-labeled fluoromicrospheres. Good correspondences were observed between CETP activity values obtained either with the ultracentrifugation method or the immunospecific scintillation proximity assay (n = 70; r = 0.94; P = 0.0001), and between values obtained for either fresh or frozen serum samples (n = 70; r = 0.93; P = 0.0001). Because of its potential for automation, the immunospecific scintillation proximity assay may constitute a convenient tool to measure serum CETP activity in the clinical laboratory.