We prepared a silk fibroin compact in which Ag nanoparticles (Ag NPs) were dispersed, by adding only silver nitrate (AgNO3) to a silk fibroin solution without using any reducing agent. FT-IR and DSC revealed that the formation of Ag NPs distorted the conformation of the silk fibroin. However, the Ag NPs did not affect the thermal stability of the silk fibroin compact. An antimicrobial test against Escherichia coli (E. coli) was performed using the silk fibroin compact in which Ag NPs were dispersed. The compact showed sufficient antimicrobial activity because the average survival of E. coli was 1.6% after 8 h incubation even when the silver content was 0.01 wt%. The addition of Ag NPs into silk fibroin compact can provide color and an antimicrobial function without spoiling the function of the silk fibroin compact. (c) 2012 Elsevier B.V. All rights reserved.