Series of microwave-absorption lines spaced uniformly in the magnetic field have been observed in single crystals of YBa2Cu3O7-δ. As the crystals are rotated, the regularly periodic lines move in field so that the component of the magnetic field along a  direction remains constant. Line series appear at successive threshold microwave powers and temperatures, and the lines widen dramatically as either the microwave power or the temperature is increased. The absorptions are believed to arise from the mixing of macroscopic flux states, stabilized by the microwave excitation. © 1988 IOP Publishing Ltd.