The DNA as isolated from duck fibroblasts transformed by a duck-adapted Prague strain of Rous sarcoma virus and used for transfection. Transformed recipient BLEF and DEF cultures exhibited considerable morphological variability. The virus designated daPR-RSV-C morphf was obtained from the culture with fusiform transformation and cloned. The virus retained the ability to induce fusiform transformation, even after 20 passages on chicken fibroblasts. There was a good correlation between focus forming activity of the virus and its tumorigenicity in chickens. The frequency of morphf mutation to another phenotype was less than 10(-3) in cloned virus. Foreign avian embryonic cells transformed by this virus clone had a similar morphological appearance as transformed chicken cells. The clone also retained two additional non-conditional markers - subgroup C specificity and the ability to replicate efficiently in duck cells ("duck adaptation"). Freshly obtained cloned virus was found not to contain a transformation-defective mutant. Such a mutant occurred in the second passage of the virus of DEF where the mutant was isolated. Inoculation of the td mutant into Brown Leghorn embryos gave rise to a sarcoma in one of the 36 examined chickens. However, no transforming virus was detected in the sarcoma. SDS-polyacrylamide gel electrophoresis showed that cloned daPR-RSV-C morphf contained only genomic RNA; its molecular weight 3.08 X 10(6) daltons corresponded to the molecular weight of a non-defective PR-RSV-C used as control.