During the past year, our efforts have been directed toward elucidation of the molecular mechanisms underlying the biologic activity of polyoma virus (py). We are continuing to characterize the viral-host DNA junctions in polyoma virus-transformed cells in order to formulate a role in integration for short areas of homology which we have observed in DNA sequences flanking the site of viral DNA integration into the host cell chromosome. Also, we have identified several species of the Py T-antigens by isoelectric focusing and are currently characterizing these both biochemically and on the basis of their reactivity to monoclonal antibodies. This year we initiated a new line of research to study the molecular basis of Py virus virulence and organ-specific tumorigenesis in newborn mice. Preliminary data indicate that these characteristics are genetically determined by alterations in a region of the genome responsible for the regulation of Py virus early gene expression.