Superconducting radio frequency (srf) cavities, essential components of many large particle accelerators, rely on the metastable flux-free state of superconducting materials. In this Letter, we present results of experiments measuring the magnetic field limits of two srf materials, Nb and Nb_{3}Sn. Resonators made using these materials were probed using both high power rf pulses and dc magnetic fields. Nb, which is the current standard material for srf cavities in applications, was found to be limited by the superheating field H_{sh} when prepared using methods to avoid excessive rf dissipation at high fields. Nb_{3}Sn, which is a promising alternative material that is still in the early stages of development for srf purposes, was found to be limited between the onset field of metastability H_{c1} and H_{sh}. Analysis of the results shows that the limitation is consistent with nucleation of flux penetration at defects in the rf layer.