Measles virus (MV) is a highly contagious agent which causes a major health problem in developing countries. Efficacious and safe live attenuated vaccine strains are available, but for the elimination of measles a better knowledge about the molecular biology of MV appears crucial. Whereas the roles of the six structural proteins in the replication cycle are known, the functions of the two nonstructural proteins C and V are unclear, which is also true for related viruses. In vitro studies implicating Sendai virus suggest that the C protein might be involved in downregulating viral mRNA synthesis (J. Curran, J.B. Marq, and D. Kolakofsky, Virology 189, 647-656, 1992). However, not all members of the Paramyxovirinae subfamily encode this protein, raising the question about its importance for the viral replication cycle. Taking advantage of a recently developed reverse genetics system allowing MV recovery from cloned DNA (F. Radecke, P. Spielhofer, H. Schneider, K. Kaelin, M. Huber, C. Dötsch, G. Christiansen, and M.A. Billeter, EMBO J. 14, 5773-5784, 1995), the question was addressed whether the C protein is essential for the life cycle of MV. A plasmid was constructed to produce a derivative of the Edmonston B vaccine strain, MV C- EdB, having its C reading frame silenced by two point mutations. The C- mutant MV could indeed be rescued, and it multiplies in cultured cells without obvious impairment.