The procurement, rearing, nutrition and microbiological monitoring of gnotobiotic pigs and a method for conditioning of primary, colostrum-deprived, specific pathogen free pigs is described. As compared to the established hysterectomy and closed hysterotomy methods for the derivation of gnotobiotic piglets an alternative approach, open caesarian section with the sow maintained under general halothane-nitrous oxide anaesthesia and the introduction of each fetus into the sterile isolator via a liquid germicidal trap, was found to be more efficient and equally successful in providing viable and microbiologically sterile piglets. Two sterile commercially available milk diets, a special formula for orphan animals and condensed cow's milk, when the latter was supplemented with injectable vitamin E, selenium and iron, proved adequate for satisfactory health of the animals. Two types of pelleted starter rations, sterilized by 4.5 megarads of gamma irradiation, provided adequately for the nutritional needs of older gnotobiotic pigs. Results of microbiological monitoring indicated that the surgical and rearing methods employed were capable of preventing contamination of the animals with bacteria, mycoplasma, yeasts, molds, protozoa and helminths but probably could not exclude occasional vertically transmitted viral infections. Exposure of the animals for four weeks to selected strains of lactobacilli, fecal streptococci and Escherichia coli did not result in visible disease while they were maintained in isolators and conditioned them for transfer into a conventional microbial environment.