Abstract4‐chloroaniline was oxidized by soya cell‐wall peroxidases in the presence of hydrogen peroxide. The main product, an orange compound with a maximal absorbance at 455 nm, was probably 4, 4′‐dichloroazobenzene. The optimum pH of the reaction was 4. Michaelis constants, determined as described by Dalziel, were 21 mM for 4‐chloroaniline and 94 μM for hydrogen peroxide. Syringaldazine was an uncompetitive inhibitor of 4‐chloroaniline peroxidation (Ki=46 μM) and modified the progress of the reaction with the appearance of a lag period. By contrast, 4‐chloroaniline was a non‐competitive inhibitor of syringaldazine peroxidation with a Kivalue of 21 mMxat pH 7.5. Therefore, these two inhibiting effects were compatible with the presence of two binding sites for two different hydrogen donors. Both sites were linked by allosteric interactions. The inferences on chloroaniline bind