A process for the production of a poliomyelitis vaccine comprises subjecting an aqueous medium containing live poliomyelitis virus to treatment with ultra-violet irradiation and, in addition, to incubation for a prolonged period at an elevated temperature and/or formaldehyde treatment in any order under such conditions that the first killing treatment kills a high proportion but not all the virus present and the subsequent killing treatment or treatments is sufficient to kill all the remaining poliomyelitis virus in the medium, none of the killing treatments of individually applied to the aqueous medium being capable of killing all the living virus present in the medium. The Specification deals mainly with poliomyelitis virus Types 1, 2 and 3 and in general each virus type is treated separately and the resulting vaccine combined later with the vaccine or vaccines prepared from the other type or types.  Any aqueous medium where the virus can be best propagated may be used to culture the virus, e.g. "199" tissue culture medium infected with poliomyelitis virus.  An example of such a medium is that prepared by macerating p  monkey kidney tissue and trypsinizing to remove extraneous tissue, the residual cells allowed to multiply, the medium inoculated with poliomyelitis virus and the mixture incubated; the fluid is then harvested.  The infectivity titer of this harvested fluid should be of the order of at least 10-5.  The ultra-violet irradiation phase of the process is carried out by exposing a thin film or stream of the aqueous medium containing the living virus to ultra-violet light of wavelength of 2000 to 3000  ngstrom units, conveniently in a centrifugal filmer type apparatus.  The film should not be thicker than 100 microns, and the ultra-violet light should emit a high proportion, e.g. 95 per cent, of energy at 2537  ngstrom units and have a total power output of 10 to 25 watts.  The light source is placed within one centimeter from the surface of the film and should provide an intensity of irradiation of from 12,500 to 3,200 microwatts per square centimetre of film surface.  The period of exposure to irradiation is preferably from 0.5 to 2 seconds in order that the vaccine may ultimately possess a high antigenicity. Optimum results are obtained if the temperature of the medium during irradiation is between 30 DEG  and 42 DEG  C.  The incubation phase of the treatment consists of heating the aqueous medium for from 2 to 20 days at 30 DEG  to 50 DEG  C. and when employed subsequent to the irradiation treatment should be started with a few hours of completing the irradiation phase. The formaldehyde phase of the treatment comprises adding aqueous formaldehyde solution to the aqueous medium containing the virus to a concentration of between 1 : 2000 to 1 : 12,000 and heating the mixture at 30-42 DEG  C. for from 2 to 6 days.  Before any one or more phases of the killing treatment, the aqueous medium may be heated at 48 DEG  to 52 DEG  C. for from five minutes to several hours, or subjected to a bacterial filtration in order to prevent aggregation of the virus particles.  Germicides and/or stabilizers may be added to the vaccine products, e.g. benzethonium chloride to a concentration of 1 : 20,000 to 1 : 50,000. The resulting vaccines may be used for the production of other poliomyelitis virus vaccine products, e.g. alum or aluminium phosphate precipitated virus vaccine products, or may be administered to mammals to induce immunity against infection by live poliomyelitis virus.  The vaccines may be administered without dilution, but usually a dilution of one to four volumes with a sterile aqueous medium, e.g. Hank's solution, is employed. Specification 655,198 is referred to.