Behavior of silver films of various origin, e.g., physical vapor deposited, electrodeposited, electroless, and metallurgical was investigated in a comparative way, using physiological solutions (0.85 % NaCl and calf serum). Only for certain thin silver films prepared by the physical vapor deposition did the open-circuit potential increase toward more negative potentials, until it reached a steady-state value in less than 100 s. These films exhibited a strong antimicrobial effect toward microorganisms such as Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus. Results in the present work suggest that the essential factor leading to antimicrobial effect is the presence of oxide(s) in the silver material. Electrochemically oxidized silver metal showed a behavior very similar to physical vapor deposited films in the open-circuit potential vs. time curves and in the antimicrobial effect. There was no evidence that pure metallic silver, no matter how it was prepared, showed a change in the open-circuit potential, nor did it exhibit an antimicrobial activity.