Intratumoral grafting of genetically engineered cells that produce interleukin-4 (IL-4) has been shown to produce tumor regression as well as prolong survival of mice harboring intracerebral gliomas. We sought to determine whether retroviral-mediated gene delivery into tumor cells in situ resulted in enhanced tumor regression by IL-4. Two mouse fibroblast lines were obtained: they both secreted similar levels of IL-4 but one produced a retrovirus vector bearing the IL-4 gene (CRE-MFG-IL-4 cells), whereas the other did not (NIH3T3-IL-4 cells). In mixed transplantation assays in the subcutaneous flanks of athymic mice, CRE-MFG, IL-4 cells were more effective than NIH3T3-IL-4 cells in inhibiting the growth of rat C6 glioma cells (p < 0.005, ANOVA). Subcutaneous tumors injected with fibroblasts that produced a control retrovirus vector without producing IL-4 (CRE-MFG-LacZ cells) did not inhibit subcutaneous tumor growth. An intracranial assay was used to evaluate survival of athymic mice harboring intracranial gliomas. Three days after implanting rat C6 glioma cells into the right frontal lobes of athymic mice, NIH3T3-IL-4 cells (n = 10) or CRE-MFG-IL-4 cells (n = 10) were stereotactically inoculated into the tumor bed. The average survival of mice treated with CRE-MFG-IL-4 cells was 38 days (+/- 2.4, SE), whereas that of mice treated with NIH3T3-IL-4 cells was 31 days (+/- 0.8, SE) (p < 0.005, ANOVA; p < 0.001, log-rank analysis).(ABSTRACT TRUNCATED AT 250 WORDS)