Résumé : Cell hybrids between malignant mouse hepatoma cells and normal rat fibroblasts with approximately one set of chromosomes from each parent exhibited remarkable karyotypic stability. Most chromosomes of both parents were retained even after prolonged culture in vitro. Normally, such hybrids showed suppression of the transformed phenotype and formed no colonies in soft agar. However, two hybrids, BS140 and BS181, formed a few colonies in soft agar when many cells were seeded, and also occasional foci of cells were detected piling up in monolayer cell cultures. We isolated soft agar colonies (a-subclones) and sub-clones from foci (h-subclones) of both hybrids, and, as a control, subclones of cells from random areas without foci of one hybrid (BS181 p-subclones). When tested for soft agar growth, cells from the a- and h-subclones of both BS140 and BS181 formed colonies at frequencies comparable to the malignant mouse hepatoma parent, whereas the control cells of the BS181 p-subclones (like the normal rat parental cells) yielded no soft agar colonies. All the cell lines were subjected to detailed karyotype analysis in G-banding, which resulted in the finding that cells from the original BS140 hybrid contained at least one copy of each rat chromosome, whereas BS140 a- and h-subclones had lost both copies of rat chromosome 5. Similarly, the original BS181 hybrid contained at least one copy of each rat chromosome, whereas BS181 a- and h-subclones displayed a deletion of the segment q22-23 of rat chromosome 5. In contrast, the control BS181 p-subclones contained one or two copies of non-deleted rat chromosome 5. The conclusion is that a gene for the suppression of anchorage independence is located in the segment 5q22-23. We propose to call this gene SAI1 (for suppression of anchorage independence). Using Southern blotting, we tested whether any of several gene probes, known to correspond to DNA sequences in rat chromosome 5, were homologous to sequences in the deletion. Only one probe, corresponding to the active alpha1-interferon gene, was shown to be located within the deletion. Hence, the SAI1 gene is closely linked to the alpha 1-interferon gene, and might be identical to this locus.