I. Introduction Parvoviruses belong to the large group of viral agents of which virologists
have become aware by chance in many biological materials due to the availabil ity of more
sensitive isolation techniques and the extensive use of the electron microscope. In general
many of these viruses lacked the stimulating background of an infectious disease and therefore
have fallen into oblivion already soon after discovery. In case of parvoviruses however
interest has been maintained because of the circumstances under which most of them were
isolated. A great number of parvoviruses has been recovered from tissues of tumor bearing
animals from cell-free filtrates of tumors or from stable cell lines of tumor origin. These
observations necessarily suggested the newly isolated viral agents of playing an important yet
unknown role in the induction and develop ment of cancer. On the other hand further
parvoviruses were found constantly associated with adenoviruses. It was the experimental
analysis of the multiplica tion behaviour which then revealed that the association between
parvoviruses and tumors or parvoviruses and adenoviruses originates from the basis of a cer
tain genetic defectiveness. For some members of the group this may be overcome by cellular
helper effects in rapidly growing tissues for several others however by biochemical events
in the simultaneously occurring replication of an adeno virus only. Additional points of view
in favour of parvovirus research have arisen from experimental animal studies.
