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Basophilic cytoplasmic inclusions, often accompanied by nuclear changes, develop regularly in primary monkey kidney cell cultures infected with influenza-B viruses. The inclusions stain positively for RNA, and electron microscopy shows them to contain large aggregates of ribosome-like particles. Inclusions form late in the infection, and it has not been possible to correlate them with the synthesis of virus-specific antigens. The capacity of influenza-B virus to give rise to inclusions is specifically neutralised by virus antibody. The inclusions seem to be a useful character for the distinction of influenza-B strains from other haemadsorbing viruses.