An experimental investigation on the use of nanofluids as working fluids and direct absorbers in a full-scale concentrating collector is presented. The nanofluid consists of a suspension of single wall carbon nanohorns in distilled water with a concentration of 0.02 g L-1. The thermo-physical properties are the same as those of the base fluid, but the presence of carbon nanoparticles greatly enhances the optical characteristics. A direct absorption receiver has been designed and set up to investigate the capability of the nanofluid to absorb the concentrated sunlight. The receiver exhibits a flat geometry and has been designed for installation on an asymmetric parabolic trough, where the concentrated solar flux locally reaches 100 kW M-2 under clear-sky conditions. Results show that the application of a carbon nanohorn-based nanofluid in a concentrating collector displays an efficiency comparable to that obtained with a surface receiver tested in the same system. However, such performance is not maintained for a long time because of lack of stability of the absorbing fluid.