Two groups of twin lambs, kept with their dams at pasture, were given 10,000 oocysts of Eimeria crandallis and 10,000 oocysts of E ovinoidalis either at birth only, or on four occasions at weekly intervals. A further group received 1000 oocysts of each species three times a week in a 'trickle infection' from birth to 21 days of age. All these lambs, together with a susceptible control group were challenged with 100,000 oocysts of each species at 28 days of age. A fifth group received no inoculations throughout. Bodyweight, faecal consistency and oocyst output were monitored up to nine weeks of age. There was no clinical response to any of the immunising inoculations and no change in the faecal consistency, but the group infected at birth grew significantly faster than the uninfected controls. The pattern of oocyst output showed that only E crandallis developed fully in the newborn animal, but both species multiplied in seven-day-old lambs. The challenge infection produced 80 per cent mortality in the susceptible control group and 20 per cent mortality in the group which had received only one immunising dose at birth. The other immunised groups were well protected and gained more weight than the unchallenged controls. At nine weeks of age, the weight gain of the lambs which had received the 'trickle infection' was significantly higher than that of all the other groups.