As the molecular masses of the protein hormones and the differences in amino acid
sequences increase, the cross-reactivity of antisera in the different species becomes more
problematic, and homologous antisera are required. Several homologous reagents are
commercially available, e.g. rat prolactin, thyrotrophin, luteotrophin and follitrophin,
canine thyrotrophin. Some specialized centres are able to produce suitable antisera given
purified hormones but this is a time-consuming process, and most toxicology laboratories