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.