In work that is complementary to our investigation of the spectroscopic features of the cytochrome c peroxidase from Paracoccus denitrificans, we have studied the kinetics of oxidation of cytochrome c by this enzyme. The enzyme, as isolated, is in the fully oxidized form and is relatively inactive. Reduction of the high-potential haem at pH 6 with ascorbate results in partial activation of the enzyme. Full activation is achieved by addition of 1 mM CaCl2. Enzyme activation is associated with formation of a high-spin state at the oxidized low-potential haem. EGTA treatment of the oxidized enzyme prevents activation after reduction with ascorbate, while treatment with EGTA of the reduced, partially activated, form abolishes the activity. We conclude that the active enzyme is a mixed-valence form with the low-potential haem in a high-spin state that is stabilized by Ca2+. Dilution of the enzyme results in a progressive loss of activity, the extent of which depends on the degree of dilution. Most of the activity lost upon dilution can be recovered after reconcentration. The M(r) of the enzyme on molecular-exclusion chromatography is concentration-dependent, with a shift to lower values at lower concentrations. Values of M(r) obtained are intermediate between those of a monomer (39565) and a dimer. We propose that the active form of the enzyme is a dimer which dissociates at high dilution to give inactive monomers. From the activity of the enzyme at different dilutions, a K(D) of 0.8 μM can be calculated for the monomer-dimer equilibrium. The cytochrome c peroxidase oxidizes horse ferrocytochrome c with first-order kinetics, even at high ferrocytochrome c concentrations. The maximal catalytic-centre activity ('turnover number') under the assay conditions used is 62000 min-1, with a half-saturating ferrocytochrome c concentration of 3.3 μM. The corresponding values for the Paracoccus cytochrome c-550 (presumed to be the physiological substrate) are 85000 min-1 and 13 μM. However, in this case, the kinetics deviate from first-order progress curves at all ferrocytochrome c concentrations. Consideration of the periplasmic environment in Paracoccus denitrificans leads us to propose that the enzyme will be present as the fully active dimer supplied with saturating ferrocytochrome c-550.