Summary. Methyl, ethyl and n‐propyl N‐hydroxycarbamates and hydroxyurea catalysed the conversion of haemoglobin into methaemoglobin in the presence of oxygen. The rate of the reaction with ethyl N‐hydroxycarbamate increased with increasing hydroxamic acid concentration. The conversion also occurred when urethane was incubated with oxyhaemoglobin in the presence, but not in the absence, of an in vitro metabolizing system.