1. The more recent literature on the experimental methods employed in variola or alastrim-vaccinia variation is reviewed and briefly discussed.2. An account is given of attempts to produce a vaccinia variant from variola of the virulent type and a successful result, by employing the method of intratesticular passage, has been described.3. The reasons for accepting this as a genuine variation and not a vaccinial contamination are discussed.4. An attempt has been made to analyse some possible factors underlying the discrepancies in the literature, and a plea is made for more co-operation in future work on the subject.