We report on third-harmonic generation (THG) of biomolecular solutions at the fluid/glass interface as a means to probe resonant contributions to their nonlinear absorption spectra that could serve as contrast mechanisms for functional imaging. Our source was $100\mathrm{fs}$ laser pulses whose center wavelength varied from 760 to $1000\mathrm{nm}$ . We find evidence of a two-photon resonance in the ratio of third-order susceptibilities, ${\chi }_{\mathrm{sample}}^{\left(3\right)}\left(3\omega \right)∕{\chi }_{\mathrm{glass}}^{\left(3\right)}$ , for aqueous solutions of Rhodamine B, Fura-2, and hemoglobin and a three-photon resonance in ${\chi }_{\mathrm{sample}}^{\left(3\right)}\left(3\omega \right)∕{\chi }_{\mathrm{glass}}^{\left(3\right)}$ for solutions of bovine serum albumin. Consistent with past work, we find evidence of a one-photon resonance of ${\chi }_{\mathrm{sample}}^{\left(3\right)}\left(3\omega \right)∕{\chi }_{\mathrm{glass}}^{\left(3\right)}$ for water, while confirming a lack of resonant enhancement for benzene. At physiological concentrations, hemoglobin in different ligand-binding states could be distinguished on the basis of features of its THG spectrum.