The authors studied the seroconversion of 307 children, 1-11 years old, induced by Mumpsvax (136 children) and a bivalent mumps-measles vaccine (strain Jeryl Lynn/strain Moraten; 171 vaccines) by the HI-test. The seroconversion induced by the measles component of the bivalent vaccine covered 96.3% of the seronegatives prior to immunization. The comparable values for mumps antibodies were lower (Mumpsvax:80.6%; bivlant vaccine: 82.2%). An increase in titer in seropositive children was obtained in 57.1% after immunization with Mumpsvax and in 50.8% with the bivalent vaccine. Comparative studies to investigate the sensitivity of the HI-test versus the neutralisation test (NT) showed the greater accuracy of the latter. Before the vaccination with Mumpsvax, only 26.6% of 60 sera studied were found to be seronegative, whilst the NT gave a more reliable figure (55%). Similar data were found in 62 sera before immunization with the bivalent vaccine (HI-test: 33.8%; NT:66.1% seronegative). The seroconversion rates were slightly higher when the NT had been used: 75.6% (bivalent) and 81.8% (monovalent vaccine) versus 75.6% and 75%, respectively, by the HI-test.