An electrophoretic mobility variant of phenoloxidase in a lz stock of Drosophila melanogaster was identified as the A3 component of the phenoloxidase complex by using two different activators to study enzyme activity-natural activator isolated from pupae and 50% 2-propanol. The structural gene for the A3 proenzyme, Dox-3, was not associated with lz on the X chromosome; it mapped to the right of rdo (53.1) and left of M(2)m in the second linkage group. The lz locus affects the differentiation of the crystal cell, the type of hemocyte that carries prophenoloxidase(s) in paracrystalline form. Alleles of lz lacking paracrystalline inclusions in their hemocytes do not have phenoloxidase activity whereas alleles with paracrystalline inclusions have enzyme activity. The presence of proenzyme in the paracrystalline inclusions was demonstrated by in situ activation with natural activator or propanol followed by incubation in buffered dopa.