This study was performed to determine the antimicrobial and antibiofilm activities of silver nanoparticles (AgNPs) biosynthesised using Streptomyces griseorubens AU2 isolated from soil. The antimicrobial activity of the AgNPs was determined by agar well diffusion, disc diffusion and broth microdilution methods. Diameters of the zone of inhibition results clearly displayed that the microbially biosynthesised AgNPs have potent antimicrobial activity against Candida albicans, Bacillus subtilis, Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus. The minimum inhibitory concentration (MIC) and minimum lethal concentration (MLC) of the nanoparticles that had been determined by broth microdilution method were found to be 20 and 50 mu g/ml for C. albicans, B. subtilis and S. aureus; 10 and 20 mu g/ml for E. coli and P. aeruginosa, respectively. For determining the effect of AgNPs on biofilm formation under in vitro conditions, MIC and subMICs were studied on P. aeruginosa and S. aureus biofilms by using microplate biofilm assay. Treatment of the AgNPs resulted in a decrease in the biofilm formation of S. aureus and P. aeruginosa as 26.52 and 25.50%, respectively. As a result of this study, it can be suggested that actinobacterially synthesised AgNPs have an effective potential to be used for pharmaceutical applications against multi-resistant microorganisms.