Morphological revertants have been isolated from one line of adenovirus type 12-transformed hamster cells. This line, T637, is oncogenic in hamsters and contains multiple copies of the virus genome per cell. Different parts of the virus genome are represented in non-stoichiometric amounts and the virus DNA persists in the cells in an integrated form. The pattern of integrated virus genomes has been determined by the blotting technique. In the T637 line, morphological revertants arise spontaneously at relatively high frequency. Two of these revertants have been cloned. In contrast to the T637 line, the revertants F10 and G12 exhibit fibroblastic morphology. The patterns of integrated virus genomes in the revertants differs markedly from that of the T637 line; one of the revertant cell lines, F10, appears to have lost all virus DNA sequences. The morphological revertants continue to express the oncogenic phenotype, although the time required to produce tumours in animals appears to be prolonged compared to the parental BHK21 and the T637 cell lines. A number of biological parameters of the revertant lines have also been investigated.