In 1898, after forty-five years of enforcing mandatory infant smallpox vaccination, the British parliament passed an act to allow parents to “opt out” of the compulsory system. The 1898 Vaccination Act introduced a conscience clause that entitled parents who objected to the practice of vaccination to claim certificates of conscientious objection by applying to a magistrate for an exemption. This provided working- and lower-middle-class anti-vaccinationists a measure of relief from the repeated fines they had suffered for noncompliance with the law, and from the threat of imprisonment. By the end of 1898, over 200,000 certificates of conscientious objection had been issued. Many of these were granted in anti-vaccination strongholds where exemptions outnumbered vaccinations, but conscientious objection to vaccination was by no means limited to these regions. Once an amended conscience clause was passed in 1907, which made conscientious objector status much easier to attain, the national exemption rate grew to 25 percent of all births.The vaccination conscience clauses were controversial. As most of the applicants who applied for these exemption certificates came from the working classes, and many were women, these acts generated a national debate over the classed and gendered nature of the conscience and the meanings of conscientious objection. The years between 1898 and 1907 thus mark a significant moment in the making of the modern subject and citizen. For, as the debate over conscientious objection to vaccination reveals, who exactly was entitled to make a claim to possess a conscience, with its concomitant rights, was itself a contested issue.