Mumps antibodies of 34 human beings were studied, 12 patients with natural mumps infection, 15 subjects vaccinated with a live mumps vaccine, and seven subjects vaccinated with an inactivated vaccine. Small amounts of antibodies reacting with mumps antigen were found in the prevaccination sera. An immunization with either the live or the killed vaccine caused an increase in the mumps antibodies (range, from 1.1-fold to more than 50-fold; geometric mean, approximately sevenfold). IgG1 was the major isotype in all post-vaccination sera; the average share was 61%. Next came IgM (28%), followed by IgA (9%), and IgG3 (2% of total). The patient samples had 10 (acute phase) or 20 times (convalescent phase) more mumps antibodies than the prevaccination samples. IgG1 was the predominant isotype in the acute phase sera (average 42% of all antibodies). Next came IgM (41%) followed by IgA (13%), and IgG3 (4%). In convalescent sera IgG1 was also the predominant isotype (average 67%), followed by IgM (19%). The minor isotypes in the second samples were IgA (12%) and IgG3 (3%). Small amounts of IgG2 antibodies were found in 1 patient and 1 vaccine. IgG4 antibodies were not detected.