A study of bovine ephemeral fever (BEF) virus was carried out using cell cultures, laboratory animals and cattle. Initially the virus strains studied were the BB7721 mouseadapted strain supplied by Dr R. L. Doherty, Queensland Institute of Medical Research; a bovine-adapted virus designated 70B/5 supplied by Mr I. Clark, Animal Research Institute, and a number of mouse-adapted strains supplied by Dr P. Spradbrow of this laboratory. These isolates all represented the 1967-68 epizootic. During the 1970-71 epizootic, 14 isolations were made in mice. These isolates were designated 71B/1 - 71B/14. During the early stages of the project, methods for isolation and assay of virus and the measurement of antibody response were developed. The susceptibility of cell cultures to virus, fluorescent antibody tracing of viral antigens, and serum neutralization procedures were all studied in detail. This was followed by a study of the pathogenesis of BEF in cattle and mice. Finally, a study of 2 aspects of economic importance to the cattle industry, the effect of BEF on semen quality and the possibility of transmitting BEF by procedures used in artificial insemination were studied. Studies on the susceptibility of cell cultures to BEF virus showed that rodent-adapted and cell culture-adapted virus could readily be grown in BHK21 and Vero cell cultures. Virus present in bovine blood preparations could be adapted directly into Vero cell cultures. However, attempts to adapt virus directly from bovine tissue homogenates to Vero or BHK21 cell cultures were singularly unsuccessful. ..........................................