Japanese quail cells transformed by the envelope-defective Bryan high-titer strain of Rous sarcoma virus [R(-)Q] were used as a source of the Rous sarcoma virus genome in three kinds of assays. (i) The simplest and most sensitive assay for infectious, endogenous viruses of the chicken belonging to subgroup E involved infection of a mixture of R(-)Q cells and turkey cells with the sample and assay of supernatants of these cells for focus formation on subgroup E susceptible cells. (ii) Inactivated Sendai virus-induced fusion of R(-)Q cells with live test cells was found to be a specific method for detection of chick helper factor. Focus formation by supernatant of the fused cells on subgroup E susceptible cells was correlated with the presence of subgroup E envelope glycoprotein on the plasma membranes of test cells. Whole blood cells as well as fibroblasts could be used in this assay. (iii) A method of assay for exogenous lymphoid leukosis viruses in which mixed cultures of R(-)Q cells and C/E cells and assay of supernatants for focus formation on C/E cells was as sensitive as assays presently used for exogenous lymphoid leukosis virus. Because no infectious Rous sarcoma virus was used as part of the procedure, the assays for infectious virus described here yielded pure pseudotypes of the input virus, an advantage for determining purity and subgroup of the input virus.