Fibroblast cell lines infected in vitro with different strains of gibbon ape leukaemia virus or the related woolly monkey virus (SSAV) synthesized two RNA species of approximately 8.4 kb and 2.9 kb. The former, a complete RNA, represents the gag-pol mRNA, while the latter is a spliced transcript lacking gag and pol, and represents the env mRNA. In contrast, RNA from one T-lymphoid cell line derived from a gibbon ape T-lymphocytic leukaemia (UCD-144) expressed a viral mRNA in addition to gag-pol and env mRNA. This RNA is 6.4 kb and lacks at least 3.0 kb of sequences derived from the internal region of the viral genome, including most or all of the pol gene. These data, as well as data from Southern blots of UCD-144 DNA, suggest that the 6.4 kb mRNA could represent a transcript from a defective recombinant provirus and may contain cell-derived sequences.