SummaryIn a model system, consistent transfection of chick embryo fibroblasts (CEF) by DNA from the XC cell line occurred, with recovery of infections Rous sarcoma virus. The techniques were then applied in attempts to recover possible human humour viruses. Even with various modifications of the XC technique, DNA from three human malignant melanoma cell lines failed to infect adult or foetal human fibroblasts, although melanoma DNA was taken up into nuclei of target cells. XC DNA did not transfect human foetal fibroblasts and melanoma DNA was ineffective in CEF. DNA from the Raji (Epstein‐Barr virus non‐producer) and QIMR‐WIL (producer) lymphoblastoid cell lines applicable techniques therefore failed to recover EB virus the putative melanoma retrovirus, or other potential tumour virus.