In order to investigate the regulatory role of the immune system on gastrointestinal macromolecular permeability in neonates, and ontogeny of the neonatal immune system, we have developed a total parenteral nutrition (TPN) program capable of maintaining immunologically "virgin" germfree (GF) neonatal piglets. GF colostrum-deprived piglets obtained by aseptic hysterectomy 3-5 days prior to term were denied any oral feeding and maintained on central venous hyperalimentation in the GF isolators. GF piglets were anesthetized and 20-gauge silicone tubing was inserted into the external jugular vein and advanced to the level of right auricle of the heart. The distal end of the cannula was subcutaneously tunneled out between the shoulders and attached to a light jacket, tether, and swivel assembly that allowed the piglet to move freely inside the cage in a germfree unit. The TPN formula provided 60 Cal/kg/day for the first day and gradually increased up to 165 Cal/kg/day for day-7 and older piglets. The GF piglets were maintained on the TPN formula for 21 days and weight gain of the GF-TPN piglets averaged 23 g/day. The hematologic and serum biochemical parameters of GF-TPN piglets were within the normal range of gnotobiotic animals of their age group. The serum levels of total protein and albumin in the GF-TPN piglets were significantly less than those of colostrum-fed piglets, who absorbed a large quantity of the colostral macromolecules through their gastrointestinal tract into the blood stream.(ABSTRACT TRUNCATED AT 250 WORDS)