Although in the absence of halide ion chloroperoxidase did not catalyze the ethylhydroperoxide (EHP)-supported oxidation of aminopyrine, in the presence of Br- or Cl-, chloroperoxidase did catalyze the oxidation of aminopyrine, generating the aminopyrine cation radical (AP+). The initial rate of AP+ formation was determined by monitoring the absorbance at 565 nm. The pH optimum of the reaction was centered around 5.0. The rate of AP+ formation showed typical Michaelis-Menten saturation kinetics with respect to EHP, aminopyrine and Br-. The rate of formation of bromine in the chloroperoxidase-EHP-Br- system was also determined by measuring the change in absorbance at 267 nm. In the system containing 1 mM EHP and 0.2 M KBr at pH 5.0, the rate was 1.8 nmol of bromine/s/micrograms of chloroperoxidase, which was slower than that of AP+ formation under the same conditions. The present results suggest that the formation of AP+ is initiated by the halogenation of the N,N-dimethylamino group followed by the homolysis of the haloammonium cation, and that the most likely halogenating reagent is an enzyme-bound halogenating intermediate.