Between 1977 and 1979, twenty thoroughbred mares each received an uterine infusion of 120 mls of mare colostrum in 380 mls of normal saline during mating heats. In 2 of the 20 mares the infusion was repeated after they had foaled normally in 1978. The mares ranged from 7 to 24 years old. In 1977, four of the six mares treated at two studs (A and B) produced live foals. During the current season (1978–79), 16 were irrigated, 6 at Stud A and 10 at Stud B. Ten were manually tested in-foal 45 days after last covering.