Complex fluids containing long polymer chains exhibit measurably large resistance to stretching or extensional flows, due to additional stresses generated by the extensional deformation of the underlying fluid microstructure. Understanding and quantifying the response of such elastic fluids to extensional flows is necessary for optimizing fluid composition for technological applications like ink-jet printing, spraying and turbulent drag reduction, as well as for fundamental understanding of polymer chain dynamics and refining constitutive equations. Using the opto-microfluidic technique of cross-slot extensional flow oscillatory rheometry (EFOR: J. A. Odell and S. P. Carrington, Journal of Non-Newtonian Fluid Mechanics, 2006, 137, 110–120), we characterize the extensional response of ultra-dilute polymer solutions, hyaluronic acid (ubiquitous in the body: synovial fluid, vitreous body of eye) and saliva. Using microlitre sample volumes and piezo-pumps for fine control over shear and extension rates, we measure the additional stress in terms of enhanced pressure drop and intensity of birefringence that results from macromolecular deformation around the stagnation point created within the cross-slot geometry. We first show that the stress-optical coefficient obtained from the slope of the birefringence vs. pressure drop curve (stress optical curve) measured in the EFOR for dilute atactic-polystyrene (a-PS) in dioctyl-phthalate (DOP) solution, corresponds well to literature values. In the limit of ultra-dilute solutions where the excess pressure drop in extensional flow is negligible, we show that it is possible to quantify the extensional response using the birefringence from the stretched chains, allowing apparent extensional viscosity measurements from a-PS/DOP solutions with as little as 2 ppm of added polymer. We also use the flow birefringence measurements to perform “hydrodynamic chromatography” and are able to reconstruct the molecular weight distribution of the closely monodisperse a-PS sample. Additionally, we characterize the extensional viscosity response and present stress optical curves for hyaluronic acid (HA) solutions and saliva, also extracting the molecular weight distributions of these fluid samples using the a-PS as a calibration standard. We believe that measuring extensional properties of biofluids using low fluid volumes in the EFOR is promising for developing diagnostic methods and improving the performance of synthetic replacements for body fluids.