The in vitro effect of post-heparin and post-heparin plus post-protamine normal serum on serum lipoprotein X (LP-X) is described. Serum LP-X is cleared after incubation with post-heparin normal serum, and serum LP-X remain unmodified when it is incubated with post-heparin plus post-protamine normal serum. The action of phospholipase A from snake venom on serum LP-X is also studied. A very small quantity of phospholipase is necessary to degrade LP-X and to transform lecithin into lysolecithin. It is concluded that phospholipase seems to be the enzyme that most likely induces the LP-X chnges after heparin administration. © 1977.