A simple and rapid method is described for labelling serum lipoproteins with 3H cholesterol. According to the tracer data as well as cholesterol analyses, the velocity of ester formation in rat sera and beta-lipoprotein-depleted human sera decreased during incubations as a first order reaction. This suggests a level of chemical activity for cholesterol far below the Km of the enzyme. The reversible inhibition by three different penicillins showed a dependency of the inhibitor concentration which supports this notion. 50 per cent inhibition of the rat enzyme was obtained with 0.1 mM dicloxacillin, 0.3 mM benzylpenicillin, and 1.4 mM methicillin (water phase concentrations). The human enzyme was about 10-fold less sensitive. The mechanism of penicillin inhibition, investigated by stripping-recombination experiments with the 20-fold purified rat enzyme chromatographed on penicillin-loaded Sephadex G-25 columns, was found to be a dissociation of the enzyme into two components, an inactive apoenzyme and a coenzyme with a molecular weight below 1000.