The reliability of RT-qPCR-based gene expression analysis critically depends on the selection of stable reference genes, among which GAPDH is one of the most widely used. In this study, we validated in silico-designed GAPDH primer pairs for their application as an internal control in peripheral blood mononuclear cells (PBMCs) and leukemia cell lines (CCRF-CEM and Jurkat). The constructed primer system demonstrated high specificity and amplification efficiency, with primer pair 5 showing optimal performance based on key design and amplification parameters. These results confirm the suitability of the selected GAPDH primers for accurate normalization in gene expression studies involving both normal and malignant blood cells.