Nine temperature-sensitive (ts) mutants of nonattenuated Edmonston strain measles virus were isolated from wild-type virus which was grown in the presence of 5-fluorouracil. Adsorption, temperature shift, and complementation experiments indicated that all these mutants were restricted at an intracellular stage of infection. However, all the mutants were more rapidly inactivated at 41 C than was wild-type virus, suggesting that the ts product of each mutant either influences or is a structural component of the virus. Three complementation groups were found to be represented among the mutants. Group A contained one mutant and it did not induce synthesis of detectable amounts of viral antigen at the nonpermissive temperature (39 C). Group B consisted of six mutants which did not induce viral antigen synthesis at 39 C and one mutant which did. Group C was represented by one mutant and it induced viral antigen synthesis at 39 C. The two mutants which induced sythesis of viral antigen also induced synthesis of relatively small amounts of virus-specific RNA at 39 C. These mutants, while producing cytoplasmic and nuclear accumulations of viral antigen at 39 C, were restricted in production of syncytia and hemadsorption. All the mutants were less neurovirulent than wild-type virus, as indicated by their inability to produce acute disease in newborn hamsters.