Human placental steroid sulphatase was partially purified from microsome suspensions of control and steroid sulphatase deficient placentae. After polyacrylamidegel electrophoresis, staining for protein and enzymatic activity revealed that steroid sulphatase from control placenta migrates at Rf = 0.44. In steroid sulphatase deficient microsomes no protein band and only a very faint sulphatase activity band could be detected at Rf = 0.44. Immunoelectrophoresis employing antisera raised against both steroid sulphatase preparations, only gave a protein precipitation line with sulphatase activity when using control placenta microsomes and the antiserum against steroid sulphatase from control placenta. All other placental microsomes-antisera combinations appeared to be negative. Our results strongly suggest that the nature of X-linked steroid sulphatase deficiency is a decreased amount of steroid sulphatase protein.