appropriate controls the concentration of each amino acid could be established. The absence of glycylproline ( < 0 . 5 m ~ ) from the samples were established by t.1.c. From each of two to six animals a minimum of four sacs were prepared from the ileum. Examination of sacs after incubation by lightmicroscopy failed to show any disruption caused by the experimental procedure. Fig. ] ( a ) shows the glycine and proline concentrations of sac contents (serosal fluid) expressed as percentages of those in sacs from uninfected control animals ( n = 15). Control values were 4.17 A 0.13 mM for glycine and 3.55 f 0. l O m M for proline. Less amino acid appeared in sacs as the infection progressed, with only a slight return towards normal within 21 days. Weight gain in infected animals was less than in controls, with a maximum difference on day 8. This correlates with the life-cycle of the parasite, which, after invading epithelial cells of the small intestine within hours of inoculation, leaves the animal around day 9 in the form of oocysts that can be detected in faeces. The extent to which the epithelial layer had recovered by day 21 is not yet known, but reports from chickens suggest that it is exceptional for macroscopic lesions to persist beyond day 12 (Turk, 1978).