DNA methylation appears to play an important role in both physiological and experimentally modified gene expression. Alterations in DNA methylation have been described in animal tumour models, in transformed cells and tumour cell lines, usually for single copy DNA sequences. By applying the isoschizomeres HpaII and MspI the methylation status of two classes of repetitive sequences (D22Z2 - a member of the alphoid repeats and D22Z3 which belongs to the HinfI repeats) in colon and stomach carcinoma were tested. A strong demethylation of both types of repetitive sequences in stomach carcinoma in comparison to healthy stomach mucosa was detected. In colon carcinoma, only minor signs of demethylation occured for the same repetitive DNA.