We have shown that polyethyleneglycols in the 414-942 Da range (PEG 600) cross the intestinal mucosa of the pig regardless of whether the mucosa is permeable or not to protein (Gut 25:520, 1984). The results indicate that PEG 600 and other low Mw markers commonly used as permeability probes (mono- or disaccharides, e.g. mannitol and lactulose, and 51Cr-EDTA) are not suitable for studying macromolecular permeability. To find a more suitable marker for testing macromolecular intestinal transmission, PEG 1000 (766-1382 Da) was gavage-fed to newborn, unsuckled (preclosure) and 36 h old suckled (post-closure) pigs together with BSA and colostrum. The blood serum levels and the urinary recovery of BSA and the PEG polymers after 2 and 4 h were determined by reversed-phase HPLC. The serum levels of the PEG:s were higher in preclosure than in postclosure piglets. An increased uptake of PEG:s greater than 1100 Da into the blood was also found in the preclosure pigs. The results indicate that PEG:s with a Mw > 1100 Da behave as macro-molecules before and after intestinal closure, since they probable follow the same route of transmission as the macromolecules. Therefore these PEG:s may be useful as markers of macromolecular permeability.