Aims: Presently, the effective antimicrobial agents have been limited by the emergence of microbial strains with multidrug resistance and biofilm formation potential. In the present study, we report remarkable antimicrobial activity of silver nanoparticles (AgNPs) synthesized from Streptomyces calidiresistens IF11 and IF17 strains, including inhibition of biofilm formation and synergistic effect of AgNPs and antibiotics against selected bacteria and yeasts. Cytotoxic effect of AgNPs on mammalian cell lines was also evaluated. Methods and Results: Analysis of biosynthesized AgNPs by Fourier Transform Infrared Spectroscopy and transmission electron microscopy revealed their spherical shape, small size in the range of 5-50 and 5-20 nm, respectively, as well as the presence of capping agents. Study of antimicrobial activity of AgNPs against Bacillus subtilis, Staphylococcus aureus, Escherichia coli, Candida albicans and Malassezia furfur evaluated by minimum inhibitory concentration (MIC) and minimum biocidal concentration (MBC) assays revealed that MICs of AgNPs from IF11 and IF17 strains against bacteria and yeasts were found to be in the range of 16-128 and 8-256 mu g ml(-1), while MBCs were in the range of 48-192 and 32-256 mu g ml(-1) respectively. AgNPs inhibited biofilm formation of microbial strains, which was tested by using crystal violet stain. The highest synergistic effect determined by fractional inhibitory index of AgNPs with antibiotic (kanamycin or tetracycline) was found against Staph. aureus; while in case of yeasts, M. furfur showed highest sensitivity to AgNPs-ketoconazole combination (FIC = 0.12). The cytotoxic activity of AgNPs towards HeLa and 3T3 cell lines was studied by MTT assay. The IC50 of AgNPs estimated against mouse fibroblasts was found to be 8.3 and 28.3 mu g ml(-1) and, against HeLa cell line, 28.5 and 53.8 mu g ml(-1) respectively. Conclusions: It can be concluded that AgNPs synthesized from S. calidiresistens IF11 and IF17 strains have potential as an effective antimicrobial and cytotoxic agent, especially when used in combination with antibiotics/antifungal agents. Significance and Impact of the Study: This study indicates potential application of biogenic silver nanoparticles as an antimicrobial agent in nanomedicine.