Abstract Superconductors (SC) have been investigated with microwaves by registering the nonlinear response (RNR) as a function of the modulation field. Equidistant narrow lines in a single YBaCuO crystal found by electron spin resonance (ESR) could be registered by RNR as absorption bands. Some narrow lines are present or absent depending on sweep direction and excursion. In granular SC, thin YBaCuO films showed critical (junction) fields H * which are smaller (3μT) than in bulk samples (5μT). They are as small as in BiSrCaCuO bulk. Using RNR with intermittent modulation, the measured complex hysteresis response indicates a spin glass type of structure of the SC network, and it demonstrates that the usual narrow band ESR registers only prepared states and not initial states corresponding to the scanning field.