A method is described for the isolation of large numbers of newborn larvae from adult Trichinella spiralis. These isolated larvae were used in experiments which tested their ability to infect mice via the intraduodenal, intravenous, and intraperitoneal routes, and to infect rats via the intraperitoneal and intravenous routes. Encysted muscle larvae were seen in diaphragm preparations from mice injected intravenously and intraperitoneally 14 days previously with newborn larvae. No muscle larvae were seen in diaphragm muscle preparations from mice which had received newborn larvae intraduodenally. Quantitative studies on larval infectivity were carried out in rats. Larval counts showed that 66% of the larvae were recoverable as mature muscle larvae from rats injected iv, while only 2% of the original dose was recovered from ip injected rats. In another experiment using larger numbers of rats in each group, newborn larvae were 73% infective in iv injected rats, and only 9% infective in ip injected rats. Nembutal, administered ip to rats prior to infection with newborn larvae, had no major effect on larval infectivity. Limited numbers of newborn larvae have been isolated from host body cavities (Matoff, 1940; Berntzen, 1965; Shanta and Meerovitch, 1967) and blood (Matoff, 1943b; Gould et al., 1955; Phillipson and Kershaw, 1961) during early phases of intestinal infection and have been described in morphological detail (Richels, 1955; Ali Khan, 1966). Denham (1967) collected parenterally infective newborn larvae after incubation of gravid females for 24 to 48 hr. However, isolation of large numbers of newborn larvae of a determinable age has not previously been described, although Larsh (1963) and others have suggested that this would be necessary in order to better correlate the stage of the parasite with host immunity. Inoculation of gravid female worms and newborn larvae of Trichinella spiralis demonstrated that newborn larvae could infect dogs and rats by the parenteral route (Matoff, 1943a). However, quantitative data about infectivity, pathReceived for publication 3 February 1970. t The Rockefeller University, New York, New York 10021. +t Please address reprint requests to author Dickson D. Despommier. * This work was supported, in part, by USPHS Training Grant F02-A-142057, USPHS Grant AI-04842, and NIH Post-Doctoral Fellowship 1F2-AI--31, 188. ogenicity, and immunogenicity of the newborn larva of T. spiralis are still lacking. We describe here a reproducible system for obtaining newborn larvae of a known age in large numbers. Isolation techniques are described, and a quantitative comparison is made of the infectivity of pure preparations of larvae inoculated into rats via the intraperitoneal and intravenous routes. We show that both mice and rats can be infected with this stage via the ip and iv routes and that the newborn larvae are more infective for the rat when they are inoculated iv as opposed to ip. MATERIALS AND METHODS Mature muscle larvae were obtained from stockinfected CFW male mice by the method of Larsh and Kent (1949). Male Wistar rats (120 g) were inoculated orally with approximately 10,000 muscle larvae suspended in a mixture of 0.6% nutrient broth and 2% gelatin with the aid of a syringe fitted with a blunt 18-gauge needle. We removed the food from the rats on the 6th day postinfection and killed them on the 7th. The entire small intestine was then removed from each animal, slit longitudinally, cut into 2-cm sections, and placed in a modified Baermann apparatus containing 0.85% saline solution at 37 C. Adult worms were collected over a period of 4 hr, and resedimented in flasks, then washed 4 times at intervals of 0.5 hr with saline at 37 C. Worms thus treated appeared free of debris and were viable. The worms were then transferred to a large, glass moisture chamber containing 300 cc of the following me-