Immunoblotting shows that the chimeric envelopes are incorporated into retroviral vector particles and are susceptible to factor Xa cleavage. Vectors incorporating the SCF-Moloney and SCF-4070A chimeric envelopes were pelleted by ultracentrifugation and subjected to denaturing sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Vectors incorporating the wild-type Moloney and 4070A envelopes were used as a control. After transfer to nitrocellulose, the blots were probed with an anti-SU antiserum and developed using enhanced chemiluminescense. The mobility of the chimeric envelopes is retarded relative to that of the unmodified Moloney and 4070A envelope glycoproteins, but becomes indistinguishable from them upon factor Xa cleavage.