SummarySpectrophotometric analysis of wild-type and mutant quail uterine extracts has shown there is much less ooporphyrin in the mutant uterus than in the wild-type uterus, both before and after the expected time of superficial pigment deposition. The data also show that after the time of expected superficial pigment deposition there is significantly less ooporphyrin in the mutant uterus than there was before. In vitro incubation of wild-type and mutant quail uterine homogenates with delta-aminolevulinic acid (ALA) produced data indicating that mutant homogenates have less capacity for ooporphyrin synthesis than do wild-type homogenates. The evidence suggests that some factor other than ALA availability prevents mutant homogenates from synthesizing as much ooporphyrin as wild-type homogenates.