Blood from 106 females (1 to 7 years old) of the Cambridge flock at the University of Wales, Bangor was analysed for 21 protein systems. Of the 11 polymorphic systems, haemoglobin protein (HBB) was found to be significantly associated with ovulation rate as measured by mean ovulation rate for all records per ewe. Mean ovulation rate for the HBB AA genotype was 1·25 ovulations higher than for the AB and BB genotypes (P < 0·001). The results suggest that a gene segregating in this breed is different from the Booroola (FecB) gene and that lambs of either sex can be screened at a young age for a major difference in ovulation rate.