The conversion of thyroxine (T4) to triiodothyronine (T3) was studied in homogenates and subcellular fractions of 10 human liver specimens obtained postmortem. Preliminary studies indicated that T45'-deiodinase activity did not decline in rat liver kept at 5oC for 6 and 24 hr after death. All human liver homogenates but one catalyzed T3production, although the quantity of T3produced varied greatly, from 8-fold in the absence of dithiothreitol (DTT) to 100-fold in its presence. The wide variation in activity found may reflect either postmortem loss or premortem decline in enzyme activity due to unrecognized nonthyroidal illness. The amount of T3produced was dependent on substrate availability, protein concentration, time, PH and temperature, and enzyme activity was greatest in the microsomal fraction. T3production was stimulated by DTT and inhibited by propylthiouacil (PTU). Thus, human liver T45'-deiodinase has properties very similar to the same enzyme in rat liver. These data suggest that results of studies of the effects of nonthyroidal illnesses and drugs on T45'-deiodinase activity in rat liver may be extrapolated to humans.