The literature is replete with references to a greater resistance in older animals than in younger ones against the establishment of nematode parasites. Experimental data confirming the existence of age resistance have been presented for a variety of nematode host systems: viz, Ancylostoma braziliense in cats and dogs (Sarles, 1929) ; Nippostrongylus muris in rats (Graham, 1932 and Chandler, 1932); Strongyloides ratti in rats (Sheldon, 1937); Trichinella spiralis in rats (Nolf and Zaiman, 1941); and Ascaridia galli in chickens (Ackert, Porter and Beach, 1935). On the other hand, there was no evidence of age resistance to Trichostrongylus calcaratus in rabbits (Sarles, 1932) or to T. colubriformis in guinea pigs (Herlich, 1958). Although there has been considerable reference to age resistance against nematode parasites in ruminants, evidence of this phenomenon is largely empirical being based primarily on field observations of naturally infected cattle. Under these circumstances, varying degrees of acquired immunity cannot be ruled out as a factor in the causation of the observed facts. Experimental data have been presented by Lucker (1952) and Goldberg (1952) suggesting that older sheep are somewhat more resistant than younger ones to infection with Haemonchus contortus and Oesophagostomum venulosum. However, Gordon (1950) stated that "age resistance appears to be practically absent as far as haemonchosis is concerned" in sheep Furthermore, Stewart and Gordon (1953) reported that 7 of 10 threeto four-yearold sheep raised worm-free died from experimental exposure to T. colubriformis. Host age appeared to have no influence on susceptibility to infection with 0. radiatum and Bunostomum phlebotomum in calves ranging from 0.5 to 1.25 years old (Mayhew, 1940) or to Cooperia punctata in cattle up to 1 year old (Bailey, 1949). On the other hand, the course of infections of calves with Strongyloides papillosus was affected by age of the host (Vegors, 1954). In rather striking contrast, however, various investigators (Bailey and Herlich, 1953; Andrews et al, 1953; and Bailey and Thorson, 1954) have incriminated gastrointestinal nematodes as the cause of severe clinical parasitism and deaths in adult cattle. As there is a paucity of unequivocal experimental evidence of age resistance to nematode parasites in cattle, some experiments were conducted in an attempt to clarify the status of this phenomenon; the results are herein reported. Some of the data from one of the experiments have been previously published in abstract form (Herlich, 1956b).