A simple and reproducible radioassay of lecithin cholesterol acyltransferase (LCAT E.C. 2.3.1.43) activity is described. The method is based on cold labelling of the serum, plasma or other LCAT and lipoproteins containing fluids with a trace of 14C-cholesterol spread in ready made sorbent discs. Since the procedure minimizes the chemical, heating and time consuming steps, it may be particularly suitable for routine investigation of the defects of cholesterol kinetics and also for experimental studies.