Twelve ewes were immunised against androstenedione (with Fecundin) from a total number of twenty four. The other twelve were used as controls. No significant difference (P<O.05) was shown by treatment in either mean number of corpora lutea or pre luteolysis blood progesterone concentrations. This suggests there is no effect of immunisation treatment upon ovulation rates of superovulated ewes. However, other work contradicts this observation. The data also suggested that blood progesterone concentrations prior to luteolysis do not influence numbers of corpora lutea. Further research is needed to understand the role of antibody titre and its influence upon ovulation rates of superovulated ewes.