Solubilization of microsomal △^5-3β-hydroxy steroid acyl transfearse(△^5-3β-OH-SAT) of rat brain and its reconstitution into liposomes were investigated. Among the detergents utilized for the solubilization, deoxycholic acid was superior to Tween 80 or Triton X-100 for the reconstituted activity of △^5-3β-OH-SAT. The enzyme activity was shown to be affected by the nature of phospholipids used for the preparation of the liposome. Phosphatidylcholines from egg yolk and soybean showed the highest activity of △^5-3β-OH-SAT and phosphatidytethanolamine came next. However phosphatidylserine and phosphatidic acid showed a lower activity than those obtained before the reconstitution. This study suggests that the presence of quaternary ammonium salt or amine group in the phospholipids stimulates the activity of △^5-3β-OH-SAT. However the presence of a carboxylic group or the absence of the amine group may have an inhibitory effect on the △^5-3β-OH SAT.