Primary cells are ideal for in vitro toxicity studies since they closely resemble tissue environment. Here, we report a detailed study on the in vitro interactions of 7-20 nm spherical silver nanoparticles (SNP) with primary fibroblasts and primary liver cells isolated from Swiss albino mice. The intended use of silver nanoparticles is in the form of a topical antimicrobial gel formulation for the treatment of burns and wounds. Upon exposure to SNP for 24 h, morphology of primary fibroblasts and primary liver cells remained unaltered up to 25 microg/mL and 100 microg/mL SNP, respectively, although with minor decrease in confluence. IC(50) values for primary fibroblasts and primary liver cells as revealed by XTT assay were 61 microg/mL and 449 microg/mL, respectively. Ultra-thin sections of primary cells exposed to 1/2 IC(50) SNP for 24 h, visualized under Transmission electron microscope showed the presence of dark, electron dense, spherical aggregates inside the mitochondria, and cytoplasm, probably representing the intracellular SNP. When the cells were challenged with approximately 1/2 IC(50) concentration of SNP (i.e. 30 microg/mL and 225 microg/mL for primary fibroblasts and primary liver cells, respectively), enhancement of GSH (approximately 1.2 fold) and depletion of lipid peroxidation (approximately 1.4 fold) were seen in primary fibroblasts which probably protect the cells from functional damage. In case of primary liver cells; increased levels of SOD ( approximately 1.4 fold) and GSH ( approximately 1.1 fold) as compared to unexposed cells were observed. Caspase-3 activity assay indicated that the SNP concentrations required for the onset of apoptosis were found to be much lower (3.12 microg/mL in primary fibroblasts, 12.5 microg/mL in primary liver cells) than the necrotic concentration (100 microg/mL in primary fibroblasts, 500 microg/mL in primary liver cells). These observations were confirmed by CLSM studies by exposure of cells to 1/2 IC(50) SNP (resulting in apoptosis) and 2 x IC(50)) cells (resulting in necrosis). These results clearly suggest that although silver nanoparticles seem to enter the eukaryotic cells, cellular antioxidant mechanisms protect the cells from possible oxidative damage. This property, in conjunction with the finding that primary cells possess much higher SNP tolerance than the concentration in the gel (approximately 20 microg/g), indicates preliminary safety of the formulation and warrants further study for possible human application.