Smears on slides taken from 44 patients suspected of having smallpox were examined for the presence of smallpox antigens and from 15 of them for chickenpox antigens also, by the fluorescent antibody method, using the conventional `sandwich' and the complement techniques. In seven, very strong non-specific fluorescence made the result unreadable. When staining for smallpox antigen results agreeing with the diagnoses established by other methods were obtained in 36 of 37, and when staining for chickenpox in 12 of 13 specimens examined. One false positive diagnosis of smallpox and one false negative of chickenpox were made. Because of intense non-specific staining no diagnosis could be made from other smears stored for three years. It was not possible to reach a diagnosis by examination of crust suspensions spread on slides. In five fresh specimens examined during the outbreak there was hardly any non-specific fluorescence and results could more easily be read. Morphological features as seen by fluorescence microscopy are described. The value of the method in the diagnosis of smallpox is discussed.
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