The virus-specific nucleotide sequences in the RNA and DNA of a Kirsten mouse sarcoma virus (Ki-MSV)-transformed non-producer human osteosarcoma cell clone and two subclones of these cells that reverted to a normal phenotype have been analysed by hybridization of sarcoma virus-specific complementary DNA (cDNA) to cellular RNA or DNA. Whereas the transformed clone had acquired de novo Ki-MSV sequences in the RNA and DNA of the cells, both the revertant cell lines seemed to have lost most or all of this information from the cellular nucleic acids. The DNA from the revertant cells lacked the sequences represented either in the Ki-MSV-specific cDNA or in the total cDNA of the leukaemia-sarcoma virus complex. Thus, the reversion of the virus-transformed human cells to normal morphology is associated with the loss of most or all of the proviral sequences from the cellular DNA.