A novel colorimetric assay method based on enzyme-induced metallization has been proposed for detection of alkaline phosphatase (ALP), and it was further applied to highly sensitive detection of avian influenza virus particles coupled with immunomagnetic separation. The enzyme-induced metallization-based color change strategy combined the amplification of the enzymatic reaction with the unique optical properties of metal nanoparticles (NPs), which could lead to a great enhancement in optical signal. The detection limit for ALP detection was 0.6 amol/50 μL which was 4-6 orders of magnitude more sensitive than other metal NP-based colorimetric methods. Moreover, this technique was successfully employed to a colorimetric viral immunosensor, which could be applied to complex samples without complicated sample pretreatment and sophisticated instruments, and a detection limit as low as 17.5 pg mL(-1) was achieved. This work not only provides a simple and sensitive sensing approach for ALP and virus detection but also offers an effective signal enhancement strategy for development of a highly sensitive nonaggregation metal NP-based colorimetric assay method.