1. 1. Somatic cell hybrids have been constructed between A9 mouse L fibroblasts and Ehrlich ascites tumor cells. These hybrids carry all or nearly all of the parental chromsomes. In mouse-swine hybrid lines constructed between mouse A9 L fibroblasts and porcine pancreatic cells the swine chromosomes tend to be eliminated after several cell divisions. 2. 2. Factor(s) responsible for the stability or elimination of chromosomes is (are) generally unknown. It is possible, however, that a modification and restriction like system may be involved. 3. 3. In a search for such a process, the enzymatic methylation of DNA in somatic cell hybrids was compared with that in parental cells. The extent of enzymatic methylation of DNA was analysed by the restriction enzymes HpaII and MspI which cleave at the sequence 5'-CCGG-3'. 4. 4. The endonucleolytic activity of Hpall is inhibited if the internal cytosine has been enzymatically methylated prior to restriction. MspI is not inhibited by this modification. 5. 5. Total extent of 5-methylcytosines in the nuclear DNA was estimated by incorporation of [ 14 C]deoxycytidine derived radioactivity in 5-methylcytosines and cytosines of DNA. 6. 6. Although no information about the extent of methylation of DNA in eliminated chromosomes could be obtained, the finding that chromosomes persisting in somatic cell hybrids contain hypermethylated DNA is compatible with the existence of a modification and restriction like system in eukaryotic cells.
