Most of the known self-assembly processes occur in solution, where nanosized objects interact each other forming new structures. Their real-time characterization in terms of the size and optical properties of these objects is vital for understanding those interactions. We report a novel application of nonlinear optics to study molecular structures and assemblies. By measuring the power of the third-harmonic generated in a solution of nanoparticles, we determined both the size and the third- order nonlinear optical susceptibility of those nanoparticles. The newly developed technique was successfully employed to observe the structural organization of collagen (type I) molecules in solution.