Adult Nippostrongylus brasiliensis were transferred to the small intestines of rats of different immunological status; previously noninfected rats (Group 1), passively immunized rats (Group 2), rats which had experienced one previous infection with larvae (Group 3), and rats which had experienced four previous infections with larvae (Group 4). Rats from each group were killed serially over 8 days following transfer and the number of worms present in their small intestines was determined. Group 1 rats displayed no significant reduction in worm burden over the period of the experiment. Group 2 rats expelled about 50% of the challenge over days 3, 4, and 5 post transfer; thereafter the surviving worm burden was unaffected. Expulsion of the adult worms was immediate in rats comprising Group 3 and 4 and continued until the entire challenge had been thrown off. The latter group showed a faster rate of expulsion and complete elimination was effected earlier. Resistance of the rat to reinfection with N. brasiliensis is manifested by a reduction in the number and size of the adult worms maturing in the small intestine, a decreased egg production, and a more rapid elimination of the worm population when compared with previously uninfected rats (Taliaferro and Sarles, 1939). It is also possible, using the serum of highly resistant rats, to transfer passively, to some extent, all the effects of a naturally acquired resistance (Chandler, 1937; Sarles, 1939). The study of resistance to reinfection with N. brasiliensis has involved, in the past, the effect of the rat's immune response to a challenge of infective larvae. A less complicated appraisal of that part of the host's immune response which brings about the expulsion of adult worms parasitizing the small intestine, is obtained from a study of the fate of an infection comprising adult worms, surgically transferred to the duodenum of the challenged rat (Mulligan, Urquhart, Jennings, and Neilson, 1965). In the present work a study was made of the "time course" of the expulsion of a population of adult worms surgically introduced into the small intestine of rats previously immunized in different ways. MATERIALS AND METHODS Preparation of adult worm inoculum and surgical procedure were as described previously (Mulligan et al., 1965). Rats were given four infections of N. brasiliensis, each rat receiving 3,000, 5,000, Received for publication 6 December 1968. 7,500, and 10,000 larvae at intervals of a fortnight. A proportion of the rats were bled by cardiac puncture 10 to 14 days following the final injection. Immune serum obtained was stored at minus 15 C until required. The remainder of the rats formed Group 4, the hyper-infected group. Rats of both sexes and of similar age, each weighing about 175 g at time of experiment, were divided into 4 groups and treated as follows: (a) Group 1 consisted of 40 rats with no previous experience of N. brasiliensis. This was the "control group." (b) Group 2 consisted of 48 rats. Each rat was weighed and passively immunized by the intraperitoneal injection of 4.0 ml of immune serum per 100 g live weight. The serum was injected in two doses, the first during the morning and the second in the evening of the day prior to challenge. The method of preparation of this anti-serum is described above. This was the "passively immunized group." (c) Group 3 consisted of 48 rats. Each rat had received by subcutaneous injection a primary infection of 3,500 infective larvae, 30 days before challenge. This was the "single infection group." (d) Group 4 consisted of 20 rats. Each rat had undergone a series of larval infections. Details of the size and frequency of the four larval infections are given above. This was the "hyperinfected group." Each rat of each group was challenged on day 0 by surgically transferring 1,000 adult N. brasiliensis to the duodenum. Several rats from each group were killed at daily intervals after challenge. The number of adult worms present in the small intestine of each rat was determined.