We performed polarization-resolved surface second harmonic generation (SHG) experiments on thin films of collagen I and IV molecules, as well as conventional CD measurements. We found that collagen IV presents little CD and no SHG optical activity, whereas collagen I exhibits large chiroptical effects involving both one-electron and excitonic coupling mechanisms. We estimated that these chiral components enhance the SHG signal from fibrillar collagen in biological tissues by typically a factor of 2. By comparing the distinct behaviors of collagens I and IV in SHG microscopy and in surface SHG experiments, we concluded that SHG microscopy is a sensitive probe of the micrometer-scale structural organization of collagen in biological tissues.