The absorption spectrum of collagen powder is measured between 610 and by time-resolved transmittance spectroscopy. Absorption spectra of breast from healthy volunteers are then interpreted, adding collagen to the other absorbers previously considered (i.e., oxy- and deoxyhemoglobin, water, and lipids). A significant amount of collagen, depending on breast type, is estimated to be present. Adding collagen to the fitting procedure affects remarkably the estimated values of blood content and oxygenation. The quantification of collagen has potential implications for the assessment of breast density and cancer risk.