In 1892, Albert L. Neisser (1855–1916), the professor of dermatology and venereology at the University of Breslau (Prussia), undertook two series of clinical experiments: one to test whether serum from syphilitic patients had a prophylactic value against contracting syphilis, and the other to test whether such serum had a therapeutic value for syphilitic patients. The first experiment was based on the erroneous scientific hypothesis called “Colles’s law,” which posited that a fetus that was syphilitic due to the father’s disease could in utero immunize its mother against syphilis. Neisser acknowledged that neither experiment achieved its goal. However, when his work was published, Neisser was criticized not for the various errors of experimental design that should have been recognizable to him, but because of his failure to obtain the consent of his non-syphilitic subjects. The political furor that followed led to the first governmental edict to require informed consent of participants in clinical research. This essay examines how Neisser’s generally forgotten investigation into therapies for syphilis led to some of the first regulations about informed consent.