Studies aimed a t determining the factors responsible for haemonchosis in cattle are reported. Observations on natural populations of Haemonchus placei confirmed the conclusions of Roberts, O'Sullivan, and Riek (1932) on the acquisition of resistance to this species and on the relationships of peak egg counts to the age of the animal and season of the year. Most of the 400 calves on which these observations were based developed a strong resistance and suffered no apparent ill effects; others only became resistant after suffering clinical haemonchosis; and about 8 per cent. either failed to develop resistance or lost it and died. Experiments with single doses, spaced doses, and continuous doses of larvae showed that H. placei has a comparatively short patent period. The trend assumed by the egg counts, irrespective of the method of infestation, was very similar, reaching its peak about the 6th to 10th week and thereafter rapidly declining to remain a t low levels. Calves on a 12 : 1 mixture of oaten and lucerne chaff gave much higher egg counts than calves on good quality lucerne alone, but the infestations did not vary in their persistence. A strong resistance was acquired, even from a single dose of larvae. This was shown by the failure of a test dose of larvae, administered after the egg count had been at low levels for some weeks, to cause any increase in the egg count; and by the presence, several weeks later a t autopsy, of relatively few adult worms, most of which were undersized with females containing few or no eggs. There were also numerous fourth stage larvae which had evidently persisted in this stage for many weeks. That so many immature worms occurred in resistant animals may signify that these young stages could be extremely important in the epidemiology of haemonchosis. "Selfcure", as defined by Gordon (1948) for H. contortus in sheep, could not be produced experimentally with H. placei in cattle, but egg count trends suggest that this type of reaction may sometimes occur among natural populations before resistance is finally acquired. Circulating antibodies could not be detected in experimentally infested animals. They were found, however, in animals with natural infestations of mixed species, and it is probable that in the latter case they were associated rather with Trichostrongylus than with Haemonchus. Data on egg counts in relation to the number of adult female worms present in experimentally infested calves strongly suggest that whilst the animal remains susceptible to infestation, egg counts are usually an accurate index of the degree of infestation, but that when resistance occurs the egg count is of little value for this purpose.