Besides its nutritional value, bovine beta-casein is a source of bioactive peptides called beta-casomorphins produced during digestion of raw or processed milk. It was shown that the beta-casomorphin 7 originates only from beta-casein variants A1 or B and may be a significant risk factor in human ischemic heart disease, arteriosclerosis, type I diabetes and sudden infant death syndrome. In this study, the frequency of the A1 allele in a sample of Polish Holstein bulls commonly used in artificial insemination (A. I. bulls) was analysed and the potential for production of milk capable of releasing undesirable beta-casomorphin 7 is discussed. Among 143 A. I. bulls, three genotypes were identified (A1/A1, A2/A2, A1/A2) resulting in frequencies of 0.402 and 0.598, for A1 and A2, respectively. Although, the clinical implications of A1 milk on human health is still under discussion it may be necessary to monitor reproductive bulls and decrease the frequency of allele A1.