In recent works, a mechanism of the wavelength-dependent nonlinearity in silica glasses volume-doped by silver nanoparticles (NPs) was studied. It was predicted that this nonlinearity can lead to the generation of supercontinuum  and solitons . In the present work, we demonstrate an experimental evidence of asymmetric spectral broadening due to the presence of NPs in composites based on WO3 host glass, and support these observations by numerical calculations. The most important property of NPs for nonlinear optical applications is their large intrinsic nonlinear coefficient. Far from the plasmon resonance, the third-order susceptibility of silver NPs in the composite is about seven orders of magnitude higher than the susceptibility of the host glass . Since the local electric field in NPs is enhanced at the plasmonic resonance, the nonlinear response of the metal can be additionally amplified as the wavelength shifts towards this resonance. Linear and nonlinear properties of the composite were evaluated using the Maxwell-Garnett model , exhibiting a strong wavelength dependence of the effective nonlinear coefficient, which determines the shape of the spectral broadening and leads to a blue spectral shift.