Using Southern hybridization with a series of probes derived from the MAV provirus we searched for structurally changed MAV-2 proviruses in the DNA from MAV-2-induced nephroblastomas. Anomalous fragments of MAV-2 provirus were found in all samples analysed in more detail. Comparison of the sizes of anomalous fragments detected by different probes in the same sample indicates that the fragments, at least in a majority of cases, do not represent recombinant proviruses (which might contain transduced oncogene) but deleted proviruses only. Various types of proviral deletions occur in different nephroblastoma clones, and no preferred type of deletion was found. The defects in MAV-2 sequences appear in infected chickens de novo, because defective proviruses also occur in tumours of chickens that have been infected with the plaque-purified MAV-2 preparations and, in addition, most of the proviral defects found prevent the virus from replicating and from being transmitted by infection. The regular occurrence of defective proviruses in tumour DNA supports the concept that the proviral defects are involved in oncogenesis. Hypotheses on the substance of this involvement are discussed.