Iduronate sulfatase of human placenta separates on DEAE Bio-Gel A chromatography into two components, a less acidic form A and a more acidic form B. The two forms have different mobilities on gel electrophoresis and different isoelectric points, pH 5.0 for form A and pH 4.5 for form B. They show the same pH optima in sodium acetate buffer and similar Km values for [3H]disulfated disaccharide substrate. Iduronate sulfatase A is more heat labile than iduronate sulfatase B. Different molecular weights were found by gel filtration while similar values were estimated by sucrose gradient centrifugation. Neuraminidase treatment of the two forms gives evidence that these enzymes contain sialic acid residues.