The level of RNA transcripts in human foreskin fibroblast (HFF) cells initiated from the avian sarcoma virus (ASV) long terminal repeat (LTR) promoter was stimulated more than 10-fold when the cells were also infected with human cytomegalovirus (HCMV). HCMV was able to stimulate transcription from the ASV LTR promoter even when all the LTR sequence upstream of the TATA box was deleted, suggesting that only the basal LTR promoter is required for the effect. There were no significant changes in the ASV RNA splicing pattern in stimulated and unstimulated HFF cells. The mRNAs showing an increase during HCMV stimulation included aberrantly spliced ASV RNA species as well as unspliced gag-pol, single-spliced env and single-spliced src mRNAs. This pattern was quite different from ASV splicing in chicken embryo fibroblasts (CEF) but typical of that seen in other mammalian cells. A dramatic increase in infectious ASV production from the normally non-permissive HFF was correlated with the increase in amount of ASV RNA in response to HCMV. Thus, there is not an absolute block to ASV production in human cells. However, infectious ASV production was inefficent in HCMV-stimulated HFF compared to that in CEF cells.