Cellular mutants defective in the expression of viral polypeptides were isolated from the Friend erythroleukemia cell line 745a by immunoselection with anti-ecotropic murine leukaemia virus serum in the presence of complement. The mode of appearance of the antiserum-resistant cells showed an interesting pattern which is discussed in the text. One of the mutants obtained contained no detectable gp70 or p15(E) while retaining gPr90env; defects appeared to reside in the processing of the gPr90env to gp70 and p15(E). In another type of mutant, gPr90env was not detected. All these mutants retained spleen focus-forming virus (SFFV)-specific gp55 and gag precursor Pr68gag. The mutants superinfected with the helper virus produced the helper and SFFV also, indicating that these mutations did not affect the replication of the exogenously infecting virus.