An enzyme immunoassay for human beta-glucuronidase was developed to determine the presence or absence of antigenically cross-reactive material (CRM) in patients with beta-glucuronidase deficiency mucopolysaccharidosis. This assay provided a sensitive means of measuring the primary interaction between the enzyme molecule and antibody but required neither pure antigen nor monospecific antiserum, an important consideration, since neither of these was available. Goat antiserum to partially purified human placenta beta-glucuronidase did not recognize differences in normal enzyme from human placenta, liver, fibroblasts, or blood platelets. CRM was identified in fibroblast extracts from all four of the unrelated beta-glucuronidase-deficient patients studied, but titration patterns indicated genetic heterogeneity among these four mutant proteins. Fibroblast enzymes from two obligate heterozygotes were distinguishable immunologically from normal enzyme. The enzyme immunoassay was also used to compare human enzyme with liver enzyme from other mammalian species. CRM was present in liver extracts of all species tested, but the liver enzymes, except for the rabbit, were weakly cross-reactive. We conclude that despite certain limitations, the enzyme immunoassay for human beta-glucuronidase is useful and that all four beta-glucuronidase-deficient patients studied possess CRM.