A rapid and reproducible method for the assay of 14C radioactivity in ketone bodies is described, based on the heat lability of acetoacetate and the ability of beta-hydroxybutyrate to quantitatively bind to and elute from an ion exchange resin. Potential interference from 14C activity in the ketone body metabolites (CO2, bicarbonate and acetone) during infusion of 14C-labeled ketone bodies is eliminated. The method was validated both by in vitro recovery studies and by the determination of rates of appearance of ketone bodies during infusion of exogenous unlabeled beta-hydroxybutyrate in dogs. The technique avoids many of the potential sources of imprecision in previous methods and makes in vivo study of ketone body turnover more practical.