Breast cancer is one of the commonest cancers among women. Conventional therapies cause adverse side effects inpatients. Cytokine immunotherapy such as interleukin-27 (IL-27) has been sought as an alternative cancer treatment inrecent years. IL-27 has been shown to improve anticancer immunity and anti-angiogenesis in cancers, however, its effecton apoptotic and anti-apoptotic gene expression especially in breast cancers is yet to be explored. Cytotoxicity of IL-27in non-cancerous (184b5) and cancerous (MCF-7 and MDA-MB-231) breast cell lines was first determined for 24-72h in this study. The results indicated that IL-27 treatment did not retard 184b5 cell growth, however, did inhibit MCF-7(48 h) and MDA-MB-231 (72 h) cell growth with IC50 at 442 and 457 ng/ml, respectively. Apoptotic (TRAIL, FADD, FAS,caspase-3 and caspase-8) and anti-apoptotic (BCL-2, AKT, and COX-2) genes were then amplified from untreated (control)and treated breast cancer cells and studied. TRAIL, caspase-3, caspase-8 gene expression was significantly (p < 0.05)upregulated in treated MCF-7 (442 ng/ml) and MDA-MB-231 (457 ng/ml) cells. Expression of FADD and FAS genes wasnot detected in both control and treated MCF-7 and MDA-MB-231 cells. COX-2 gene was also not expressed by MCF-7cells, but reduced significantly (p < 0.05) in treated MDA-MB-231 cells. In MDA-MB-231 cells, IL-27 treatment seemedto slightly enhance the expression of AKT and BCL-2 genes which, on the other hand, was downregulated in treatedMCF-7 cells. Conclusively, IL-27 is able to inhibit breast cancer cell growth and regulate apoptotic and anti-apoptoticgene expression in breast cancer cells.