A model of an enzyme reaction cycle that includes the generation of a transient spin-correlated radical pair state is discussed. The recombination yield of the radical pair state is altered by external magnetic fields (radical pair mechanism). In this theoretical study, the response behavior of the enzyme to pulsed magnetic fields as well as combinations of static and sinusoidally oscillating magnetic fields is investigated by using an approach that combines enzyme kinetics with magnetic field-sensitive spin kinetics. Calculations show that the enzyme behaves like a frequency sensor that is responsive at lower field frequencies but less responsive at frequencies that are faster than the time scales inherent to the kinetic properties of the reaction cycle. There is a characteristic transition region in the frequency domain that reflects the enzyme's relaxation behavior to time-dependent external perturbations. The transition region is characterized by using methods based on the theory of externally driven systems, including Floquet theory and the calculation of correlation functions. Model simulations suggest that time-dependent magnetic fields could be used as a tool to study the response behavior of magnetic field-sensitive enzymes. (C) 1997 American Institute of Physics.