Examination with the electron microscope showed that mitochondrial and microsomal fractions from py human placenta were not homogeneous, despite their careful preparation by several methods. Repeated washing of the mitochondrial fraction, which substantially reduced its glucose-6-phosphatase activity, caused only a small decrease in the aromatizing activity of this fraction. In incubations with 4-androstene-3,17-dione, testosterone and 3Β-hydroxy-5-androstene-17-one, the ratio of 17Β-estradiol to estrone produced was greater with the microsomal ( > 2.00) than with the mitochondrial fraction (0.59–1.82) at each substrate level.