Complex concentration-dependence of surface-enhanced Raman scattering (SERS) and UV–Vis absorption of Ag-nanoparticles (AgNPs) mixed with Gly has been observed. Surprisingly, with decreasing Gly concentration, a new band in UV–Vis absorption of AgNPs/Gly mixtures is found to red-shift with increasing intensity, until a turning point at a critical concentration. Further diluting Gly, the new band blue-shifts with decreasing intensity. Similarly, the SERS intensities of Gly bands at 615 and 905 cm–1 consistently increase with decreasing Gly concentrations, reaching maxima at the critical concentration. This agrees consistently with the variation in position and intensity of the new developing plasmon absorption band. Interestingly, transmission electron microscopy (TEM) revealed Gly-induced modifications of AgNPs, including a reassembling and increasing aspect ratio with deceasing Gly concentration. The concentration-dependent behavior of UV–Vis absorption, SERS, and TEM of AgNPs/Gly mixtures could be due to the complex nature of Gly-AgNPs interaction depending on the molecular density, as supported by TEM images. Copyright © 2012 John Wiley & Sons, Ltd.