Our group and others have recently demonstrated the ability of recombinant baculoviruses to transduce mammalian cells at high frequency. To further characterize the use of baculovirus as a mammalian gene delivery system, we examined the status of transduced DNA stably maintained in Chinese hamster ovary (CHO) cells. Four independent clones carrying two introduced markers, the genes for neomycin resistance (Neo) and green fluorescent protein (GFP), were selected. PCR analysis, Southern blotting, and DNA sequencing showed that discrete portions of the 148-kb baculovirus DNA were present as single-copy fragments ranging in size from 5 to 18 kb. Integration into the CHO cell genome was confirmed by fluorescent in situ hybridization (FISH) analysis. For one clone, the left and right viral/chromosomal junctions were determined by DNA sequencing of inverse PCR products. Similarly, for a different clone, the left viral/chromosomal junction was determined; however, the right junction sequence revealed the joining to another viral fragment by a short homology (microhomology), a hallmark of illegitimate recombination. The random viral breakpoints and the lack of homology between the virus and flanking chromosomal sequences are also suggestive of an illegitimate integration mechanism. To examine the long-term stability of reporter gene expression, all four clones were grown continuously for 36 passages in either the presence or absence of selection for Neo. Periodic assays over a 5-month period showed no loss of GFP expression for at least two of the clones. This report represents the first detailed analysis of baculovirus integrants within mammalian cells. The potential advantages of the baculovirus system for the stable integration of genetic material into mammalian genomes are discussed.