Two independent clones of fetal mink lung cells (CCL64) nonproductively transformed by Abelson murine leukemia virus (Ab-MuLV) were used to study spontaneous reversion to the nontransformed phenotype and subsequent retransformation of the revertants. One clone, D62, contained two complete Ab-MuLV proviruses and expressed polyprotein P120. The other clone, K49, contained four proviruses: three of them were complete and one represented a deletion mutant. In addition to P120, a new polyprotein, P60, was expressed in this clone. During the processes of reversion and retransformation proviral DNAs were conserved with respect to size and integration site. In contrast to the transformants, expression of Ab-MuLV P120, and in case of clone K49 also of P60, was blocked in revertant lines as a result of loss of transcription of proviral DNA. In retransformants, expression of Ab-MuLV P120 was found in both clones. However, no expression of P60 was detectable in retransformants of K49-derived revertants. Reversion to the nontransformed phenotype was associated with increased cytosine methylation in proviral DNA sequences, whereas in spontaneous retransformants methylation tended to resume control levels. These findings demonstrate regulation of viral oncogene mediated transformation by cytosine methylation and suggest that transcription of proviral DNA is under both viral and cellular control. They furthermore suggest that processes involved in regulation of proviral expression do not affect all such proviruses simultaneously in the same way.