G. Kostner and A. Holasek abstract: The composition of human chyle chylomicrons was studied. For thispurpose, thoracic duct chylomicrons from a female subject were isolated, delipidized, and the lipid and protein part investigated separately. For the character-ization of the protein part, polyacrylamide gel electrophoresis, immunoelectrophoresis, and immunodiffusion were per-formed. Pure peptides isolated from human serum lipo-proteins were used as reference substances. Monospecific antibodies were used to check the identity of chylomicron pep tides. In order to quantitate the amount of different constituents, a densitometric scan after separation in polyacrylamide gels and staining with Amido-Schwarz and coomassie blue were performed. In other experiments, the c^ chylomicrons are triglyceride-rich lipoproteins of diam-eter> 800 A, found in intestinal lymph after absorption of dietary fat (Havel, 1970). There is evidence that all classes of lipoproteins can be synthesized in the mucosal cells of the