Iduronate 2-sulphate sulphatase (EC 3.1.6.-) was found in human placenta in three forms which could be separated by elution from DEAE-cellulose using an NaCl gradient. Form C, most firmly bound to DEAE-cellulose, was 40% larger than the other two (forms A and B in order of ease of elution from the ion exchanger). Forms B and C contained sialic acid which could be removed by neuraminidase digestion. After removal of sialic acid form B became indistinguishable from form A. The enzyme forms found in placenta were compared with those from other human tissues and fluids by means of DEAE-cellulose chromatography and gel chromatography. Serum and amniotic fluid contained only form C, urine and cultured fibroblasts contained the less-anionic forms as well, and kidney contained appreciable amounts only of form A. Pre- and post-natal diagnosis of the Hunter syndrome both involve measurements on the enzyme which is present in form C. This is not accompanied by less-anionic forms which constitute the bulk of the enzyme as it is isolated from easily available sources such as urine.