The detection of virus antigens in tissue-cell monolayers by immunofluorescence is now an established procedure, as are the extensions of such procedures to the assay of virus infectivity by the counting of fluorescent cells. Assays have been reported for a number of viruses including the arboviruses yellow fever (Casals group B; Hahon, 1966) and Venezuelan equine encephalomyelitis (Casals group A; Hahon & Cooke, 1967). The present note is a contribution to the assessment of immunofluorescent methods in the assay of group A arbovirus infectivity and it describes the rapid counting of infected-cell foci formed by Semliki forest virus (Smithburn & Haddow, 1944) in monolayers of HeLa (Appleyard & Westwood, 1964), BHK21 (Stoker & Macpherson, 1964) and primary chick embryo cells. The data are compared with those obtained by the much slower plaque-assay procedure in suspensions of embryo cells in agar. The immunofluorescent staining of Semliki forest virus in tissue-cell culture or animal tissue has been described by Taylor (1965), Mims, Day & Marshall (1966) and Erlandsen (1967).


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