LITTLE is known about the reproduction cycle of mumps virus. This seems strange, as some of the techniques available for influenza virus should be applicable to this virus, which is in many ways similar in its reactions. The method introduced by Hoyle1,2 in his study on the multiplication of influenza virus in egg membranes, however, did not yield clear-cut results with mumps virus. The main difficulties were the slow adsorption of the virus to the reproductive cells, the relatively high proportion of free (vital) virus particles not taking part in the reproduction process during the experiments and the relatively low reproduction velocity. This last factor made it necessary to expand the experiments to more than thirty hours, and consequently the experiments had to be divided into many parts. The comparability of the infection-, haemagglutination- and other titres is impeded by this division of the experiment.