Retrovirus vectors only integrate into the genome of dividing cells and can thus be used to selectively infect tumor cells in the adult rat brain. Gene delivery was assessed by using the retrovirus BAG vector, which bears the Escherichia coli lacZ gene under the MoMLV LTR promoter-enhancer element, and by histochemical staining for bacterial beta-galactosidase activity. Direct injection of this vector (90-900 cfu) into the adult rat brain, with or without prior inoculation of C6 glioma cells (2 x 10(5) cells) resulted in labeling of only a few cells as assessed 1 week later. When the psi 2-BAG packaging line was grafted into the brain, labeled psi 2-BAG cells could be found after 1 day, but not after 5 days, following grafting, suggesting that the grafted cells had been rejected and that no endogenous cells had integrated released vector, or that expression of lacZ had been turned off. In contrast, when the psi 2-BAG packaging line was grafted into a brain region, which had been inoculated previously with rat C6 glioma cells (2 x 10(5) cells), beta-galactosidase labeling of these tumor cells, identified by immunocytochemistry for glial fibrillary acidic protein and S100, could be demonstrated 10 days later. Thus, grafting of retrovirus packaging lines into adult brain provides a mean to infect tumor cells in situ. The grafted packaging cells may continue to release retrovirus particles for an extended period, thus infecting more cells at the stage of division appropriate for viral integration, as compared to inoculation of the virus alone. Grafting of retrovirus packaging cell lines could be used to selectively deliver "killer" or "suppressor" genes to tumor cells in the brain to curtail their growth.