A patient admitted to this hospital with Parathion poisoning had a marked reduction in serum cholinesterase activity and concentration of low density lipoprotein. The enzyme and the lipoprotein levels returned to normal with clinical recovery. Guinea pigs treated with phospholine iodide showed depressed levels of serum cholinesterase and beta-lipoprotein. It is suggested that the decrease in beta-lipoproteins is due to the decrease in cholinesterase activity.