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SUMMARYVaccinia virus has been examined with the electron microscope by several techniques in conjunction with experimental modifications of structure. Negative staining revealed that over 80 yo of the virus in highly purified preparations were particles which appeared to have a beaded surface like a mulberry and were termed M forms. The beading was formed by loops of thread-like structures 9 0 6 wide which were themselves double helices formed from two 8oA strands coiled to a 1206 pitch.Twenty per cent or less of the particles appeared as larger more electrondense bodies with a capsule of complex structure; these have been termed C forms. Experimental interconversion of the two forms showed that both were structurally mature virus. The relationship of structures seen in C forms to those revealed by thin sectioning is considered. Experimental findings and the results of shadowing and replica examinations indicate the presence of a superficial protein layer of antigenic material not revealed by negative staining. The relationship of this layer to the structure of the virus and the nature of the ribbon structure of the M form are discussed.

SUMMARYFollowing intraperitoneal infection by an avirulent strain of Semliki Forest virus, athymic nude mice showed almost normal clearance of viraemia and a transitory peak of antibody activity at 5 to 9 days which fell to less than about o.r ~o of the normal antibody activity from the I4 th day.

SUMMARYSoluble antigens extracted from rabbit skin infected with vaccinia virus produced immunodiffusion patterns containing up to 17 lines. At least 5 of the components which produced lines were labile when heated at 60". Soluble material obtained from purified vaccinia virus particles produced 8 precipitin lines and an additional virus component was detected with antiserum prepared against inactivated virus particles. Seven of the virus particle precipitin lines were identical with soluble antigen lines, but up to 10 of the soluble antigen lines did not appear to represent components of the virus particle. It is suggested that they represent specific substances (perhaps enzymes) required for virus replication but not incorporated into the virus.

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