It is only recently that the long-standing assumption of a viral etiology of rubella was substantiated by the isolation, successful propagation in tissue culture and immunological identification of the responsible agent (1--3). Weller and Neva (3) described a specific, mild and limited cytopathic effect but otherwise the main manifestation of infection of tissue cultures was the development of resistance to snperinfection with ECHO 11, or Sindbis virus. The present communication reports preliminary results of electron optic studies on infective tissue culture material.Our strain of virus was kindly provided by Dr. J. L. Sever, NationalInstitutes of Health, Bethesda, Md.~ Isolated from the throat of a 6 year old boy, it was received in its 5th passage in grivet monkey kidney cultures. Material from 5 further passages in the same type of cells was used in the present study. Demonstration of virus growth by challenge infection with ECHO 11 virus, virus titrations and neutralization tests were performed mainly according to the technique described by Parkman et al. (1, 2); details will be published later. Titers of about 104 TCD~0/ml. were obtained. Preliminary tests indicated a very low density of the virus, since it did not sediment in a sucrose solution of density 1.12 g/m1. Accordingly preparation for electron microscopy was carried out as follows.Tissue cultures were harvested about l0 days after inoculation. After two cycles of freezing and thawing the fluid was clarified by centrifugation (20 minutes at 1000 r. p. m.) and

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