A systematic study on the in vitro interactions of 7-20 nm spherical silver nanoparticles (SNP) with HT-1080 and A431 cells was undertaken as a part of an on-going program in our laboratory to develop a topical antimicrobial agent for the treatment of burn wound infections. Upon exposure to SNP (up to 6.25 microg/mL), morphology of both the cell types remained unaltered. However, at higher concentrations (6.25-50 microg/mL) cells became less polyhedral, more fusiform, shrunken and rounded. IC(50) values for HT-1080 and A431 as revealed by XTT assay were 10.6 and 11.6 microg/mL, respectively. When the cells were challenged with approximately 1/2 IC(50) concentration of SNP (6.25 microg/mL), clear signs of oxidative stress, i.e. decreased GSH ( approximately 2.5-folds in HT-1080, approximately 2-folds in A431) and SOD ( approximately 1.6-folds in HT-1080, 3-folds in A431) as well as increased lipid peroxidation ( approximately 2.5-folds in HT-1080, approximately 2-folds in A431) were seen. Changes in the levels of catalase and GPx in A431 cells were statistically insignificant in both cell types. DNA fragmentation in SNP-exposed cells suggested apoptosis. When the apoptotic thresholds of SNP were monitored with caspase-3 assay the concentrations required for the onset of apoptosis were found to be much lower (0.78 microg/mL in HT-1080, 1.56 microg/mL in A431) than the necrotic concentration (12.5 microg/mL in both cell types). These results can be used to define a safe range of SNP for the intended application as a topical antimicrobial agent after appropriate in vivo studies.