Two groups of sheep born and raised worm-free were dosed with 3000 infective Haemonchus contortus larvae. Group 1 were each given a single dose of infective larvae on day 0 while the group 3 animals were dosed with 100 infective larvae per day for 30 consecutive days. The results of faecal worm-egg counts performed on alternative days, and of differential worm counts carried out on animals slaughtered at crucial times during the course of infection, showed that retardation of development at the 4th larval stage occurred in the group 3 infections and was the major effect of control of the parasitic burden. There was no evidence that egg production per female was affected.

Three animals of the nineteen infected in group 1 died during the course of the experiment owing to the pathogenic effects of the worm burden. Deaths did not occur among the animals of group 3. This finding was correlated with the observation that haematocrit levels were consistently lower among the animals of group 1 than among those of group 3.

The results are interpreted in terms of the theory of threshold behaviour of the immunological response to parasitic infection.

We are greatly indebted to Dr D. F. Stewart for his interest and constructive criticism during the course of these studies, and to Mr E. Teleki and Misses Helen Giller and Lindy Stothart for their valuable technical assistance.