Field trials were run to evaluate the commercially available preparation Fecundin (Cooper/Glaxo) which is a hormone immunoneutralization technique for boosting lambing percentage. Fecundin is a vaccine produced by linking androstenedione with a protein to form a conjugate which when formulated with an adjuvant stimulates an active immune response within the ewe. Antibodies produced against the vaccine bind with naturally occurring androstenedione present in the blood leading to a moderation of feedback mechanisms which suppress follicular development. The result is higher ovulation rate leading to production of more lambs. Fecundin was developed and widely tested in Australia and New Zealand. The objective of these trials was to evaluate the levels of response obtained in a range of common U.K. sheep breeds under farm conditions, to monitor the effects of weight changes around mating on this response and to make an economic assessment of this technique of increasing lambing percentage.