A description of the elementary steps of the horseradish peroxidase (HRP)-catalyzed oxidation of NADH is presented, along with a quantitative analysis of the magnetic-field dependence of the enzymatic reaction. In the absence of H2O2, the catalytic cycle begins with single-electron transfer from NADH to native HRP to form the NADH(center dot+) radical cation and the ferroperoxidase intermediate (Per(2+)). The theoretical framework for the magnetic-field dependent recombination of radical pairs has been extended to describe the magnetic-field dependence of reaction rate constants for multi-spin paramagnetic pairs, including the NADH(center dot+) radical cation and Per(2+) that exist in a correlated quartet electronic spin state. Good agreement between the experimentally observed and the theoretically calculated magnetic-field dependences of the effective rate constants underlines the importance of the initial single-electron-transfer step and supports a model in which the catalytic cycle begins with the one-electron reduction of HRP by NADH.