Hamster cells transformed with the Schmidt-Ruppin strain of avian sarcoma virus were selected for resistance to ethidium bromide (EB). The resistant cell lines proliferated in the presence of up to 30 µg/ml EB.

From avian sarcoma virus-transformed hamster cells already resistant to bromodeoxy-uridine (BrdU), ethidium bromide-resistant cells which were able to grow in 10 µg/ml EB were also prepared. These cells remain deficient in thymidine kinase activity and are suitable for selective preparation of hybrid cells.

The EB resistance was genetically stable. The EB-resistant cell lines, and doubly resistant cells (BrdU, EB) showed no differences in mitochondrial ultrastructure compared with the original cell lines. Thymidine incorporation into mitochondrial DNA was not influenced by EB resistance.

All resistant cell lines, including the doubly resistant cell line, contained the avian sarcoma virus genome. The number of cells needed for positive rescue experiments for avian sarcoma virus genome by cell fusion with permissive chicken embryo cells was the same as with the original cell lines. The single EB-resistant cell lines contained R-type virus-like particles, while in BrdU-resistant and doubly resistant cells the R-type particles were absent.

The possible nature of EB resistance is discussed.

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