Ascorbic acid inhibits both of the H2O2-myeloperoxidase-halide reactions which have been implicated in the bactericidal activity of neutrophils. Both iodination of zymosan in intact neutrophils and the decarboxylation of L-alanine by neutrophil sonicates do not take place in the presence of 0.01 m ascorbic acid. Despite the inhibition of these reactions, the clearance of Staphylococcus aureus and Escherichia coli in intact phagocytes is normal. The mechanism by which ascorbic acid inhibits H2O2-myeloperoxidase-halide reactions is unknown.