Summary: Twenty-five cases of clinical measles were investigated using tissue culture techniques for isolation and identification of the causative agent. Isolation of measles virus was successful in 9 early cases. In all patients from whom the agents could not be isolated the failure was correlated with the presence of serum antibody that emerged soon after appearance of rash. Propagation of the agents recovered and neutralization tests were carried out in HAM tissue. The tissue culture behavior and immunologic properties of 9 measles virus agents investigated were indistinguishable. Immunologic relationship of 6 agents was examined by cross-neutralization tests. Neutralization and complement-fixation techniques have been utilized to measure the antibody response to infection, the persistence of antibody in children recovered from the disease, in adults, and the relationship of antibody level in cord and maternal blood. The neutralizing antibody level in different lots of γ-globulin was also determined.