Polyclonal antibodies were obtained from rabbits by injection of iduronate sulfatase purified 35,000-fold from human placenta, after elution of the enzyme from sodium dodecyl sulfate (SDS) polyacrylamide gels. The specificity of these antibodies towards iduronate sulfatase was demonstrated by immunoprecipitation of enzyme activity; the level of other lysosomal hydrolases and sulfatases remained constant. Immunoblot of iduronate sulfatase from various human sources showed that the antibody recognises a polypeptide of mol. wt. 72,000 daltons in placenta and serum, and a form of mol. wt. 60,000 daltons in fibroblasts. No immunoprecipitable peptide was found in urine or in the culture medium of fibroblasts. Polypeptides of the same molecular weight were recognised in serum and in fibroblasts of Hunter patients. The presence of altered proteins in these patients was also shown by competition experiments. The addition of Hunter proteins alters the binding of normal enzyme to the antibody.