Two freshly isolated mumps virus strains were adapted to embryonated eggs. The activity of their neuraminidase (NA) and their antigenicity were examined in the 5th, 10th and 15th passages. The NA was characterized by estimation of the Michaelis constant, while the antigenicity of virus strains was assessed by induction of antibodies as detected in complement fixation (CF) tests. The KM value for strain Berlin 9/76 NA in its 15th passage was 5.65 mmol/l and exceeded 4.2 times that of strain Berlin 1/76 (KM = 1.31 mmol/l). The KM values of both strains increased in the course of passaging by 55 or 260%, respectively. The antigenicity of both egg passaged strains remained unchanged. Under the same conditions, the Jeryl Lynn vaccine strain revealed a seroconversion rate of 44%; the KM value of the NA from this strain was closely similar to that of Berlin 1/76 virus in the 15th passage (= 1.34 mmol/l).