Immature and mature laboratory mice of both sexes were exposed to Rhipicephalus sanguineus larvae. The former were significantly more susceptible than mature mice. Male mice in both age-groups and immature females became infested with significantly more R. sanguineus larvae when treated with cortisone than did control mice. Mature male mice treated with cortisone became as susceptible to infestation as untreated immature mice, but the naturally more resistant mature female mouse became even more resistant upon cortisone treatment. Estrogen-treated male mice became more resistant than controls in both age-groups. It appears that female mice are more resistant to infestation because of some factor associated with the female hormone level. Almost all helminthic and protozoan infections which have been examined experimentally become enhanced upon cortisone treatment of the host. This has been attributed to an anti-inflammatory effect and a lowering of the host's resistance rather than modification of the parasites themselves. Work on ectoparasites appears to be confined to the sheep ked, Melophagus ovinus, and the rabbit flea, Spilopsyllus cuniculi. The observations of Nelson (1962), Nelson and Bainborough (1963), and Rothschild and Ford (1964) on these insects demonstrate that modification of the host with cortisone also increases the susceptibility of the host. Among ectoparasites the significance of the age of the host in its susceptibility appears to have been noted only in a few cases. Buxton (1948) observed that Xenopsylla cheopis that fed on very young mice laid fewer eggs than those fed on adult mice. Rothschild and Ford (1964), in their work on Spilopsyllus cuniculi, noted that 2to 6-week-old rabbits were unattractive to these fleas, while very young rabbits (5 or 6 days old) and does during the last 10 days of pregnancy were the most attractive. In Minnesota, Cook and Beer (1958) noted in wild populations of meadow voles, Microtus pennsylvanicus, that old males harbored more lice (Hoplopleura acanthopus) than young male animals. This was not true in female Received for publication 16 August 1965. *Present address: Department of Parasitology, The College of Veterinary Medicine, University of Baghdad, Iraq. meadow voles or in deer mice (Peromyscus maniculatus bairdii) infested with H. hesperomydis. This infectibility of a host with certain ectoparasites appears to be related to its age, but in different ways in different host-parasite relationships. The sex of the host and its effect on different ectoparasitic infections has received some attention. Cook and Beer (1958) found sigificantly more male than female Microtus pennsylvanicus infested with the louse Hoplopleura acanthopus, but no difference attributable to sex was observed in the case of H. hesperomydis on deer mice. Rothschild (1964) states that bird fleas of the family Ceratophyllidae and various fleas on small mammals are generally more numerous on male than on female hosts. The human flea, Pulex irritans, however, according to the same author, shows a predilection for women; and bat fleas in the genus Ischnopsyllus move onto female bats before the bats migrate to their summer colonies. Among the acarines, only Nutting (1964) appears to have noted differences related to the sex of the host. Demodex spp. infested more males than females in the hamster, Mesocricetus auratus, while the reverse was true in the rabbit, Sylvilagus transitionalis, and bat, Myotis lucifugus. The experimental use of sex hormones and surgical procedures in the host, although widely used for internal parasites, have not been applied for ectoparasites. The purpose of the present study was to assess experimentally the effect of age, sex, estrogen, and cortisone in the laboratory mouse