Investigators studying the molecular biology of Semliki Forest virus and Sindbis virus, two alphaviruses (previously group A arboviruses) met recently at the First International Symposium on Arboviruses in Helsinki. They proposed the following nomenclature for the virus-specific polypeptides formed in cells infected by these and similar viruses. Proposed nomenclature for virus-specified polypeptides * From previous published reports the notation E2 and E3 are the cleavage products of the cellular precursor protein NVP 62–68 for Semliki Forest virus; for Sindbis virus, E2 is the product of the cellular Precursor noted as PE2. † A relationship between the amino-acid sequences of a polypeptide and those of virus structural polypetides should be determined, before the p-molecular weight terminology is used.