Production and expression of double-expression vectors which transduce both Neo(r) and lacZ genes and are based on the structure of avian leukosis virus were enhanced by using cis-acting sequences (long terminal repeats and noncoding sequences) from Rous-associated virus-1 and Rous-associated virus-2 rather than those of avian erythroblastosis virus previously used in our constructs. Polyclonal producer cells obtained after transfection of these vectors into the Isolde packaging cell line gave rise to titers as high as 3 x 10(5) lacZ CFU/ml, whereas it was possible to isolate clones of producer cells giving rise to titers of more than 10(6) resistance focus-forming units per ml.