1. Two pro-(phenol oxidase) were distinguished when the blood of late-third-instar larvae of Calliphora erythrocephala was electrophoresed in polyacrylamide gels with Tris-glycine buffer, pH 8.3. One pro-(phenol oxidase), after activation by an enzyme readily catalyses the oxidation of both L-tyrosine and L-3,4-dihydroxyphenylalanine (L-dopa). The second enzyme catalyses the oxidation of L-dopa but not of L-tyrosoine. 2. One of the pro-(phenol oxidases) was purified over 2000-fold from homogenates of whole larvae. This enzyme, after activation, catalyses the oxidation of both dopa and tyrosine. On electrophoresis in polyacrylamide gels with Tris-glycine buffer, pH 8.3, it has the same mobility as the enzyme in the blood which catalyses the oxidation of both tyrosine and dopa. 3. The pro-(phenol oxidase)-activating enzyme was purified over 100-fold from homogenates of whole larvae. 4. The oxidation of L-tyrosine, in the presence of the activated purified phenol oxidase, reached a steady maximum rate after a lag period that was directly related to tyrosine concentration and inversely related to enzyme concentration. 5. The effect of the addition of electron donors on the lag period was studied. Dopa, dopamine (3,4-dihydroxyphenethylamine) and 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydropteridine are the most effective hydrogen donors. 3,4-Dihydroxybenzoic acid, the oxidation of which was not catalysed by the activated pro-(phenol oxidase), did not affect the lag period.