One strategy to achieve efficient gene delivery into brain tumors employs the stereotactic implantation of fibroblasts that express a foreign gene and produce a retroviral vector bearing that gene. Another method involves the grafting of fibroblasts genetically engineered to produce a foreign gene product of interest. It is not clear to what extent retrovirus production in vivo provides an advantage over the grafting of genetically engineered cells for the purpose of achieving transgene expression. These two methods of gene delivery were compared in vivo by using the following cell lines: CRIP-MFG-LacZ cells, which express the lacZ gene and produce retrovirus vectors that bear this gene, and CRIP-LacZ cells, which express the lacZ gene, but do not produce retrovirus. Gene delivery was assessed in C6 gliomas established in the righ frontal lobe of athymic mice. CRIP-MFG-LacZ or CRIP-LacZ cells were inoculated stereotactically into these tumors. When CRIP-MFG-LacZ cells were used, a relatively elevated level of lacZ gene expression was present in cells scattered throughout the tumor. Using a computerized imaging system, this expression occurred in approximately 10% of the tumor area at 1 week, 42% at 2 weeks, and 32% at 3 weeks. In contrast, with CRIP-LacZ cells, lacZ gene expression was much weaker and occurred in a more focal area within the tumor. This expression occupied approximately 5% of the tumor area at 1 and 2 weeks and had almost disappeared at 3 weeks. In both cases there was no notable expression of the transgene in normal brain cells. In conclusion, transgene expression in brain tumors was achieved in more cells, at higher levels, and for longer time periods with retroviral vector-producing cells than with genetically engineered fibroblasts. This efficiency of gene delivery likely results from direct in situ delivery of the transgene to tumor cells with subsequent inheritance of the reporter gene to progeny tumor cells.