Arylsulfatase C and steroid sulfatase were thought to be identical enzymes. However, recent evidence showed that human arylsulfatase C consists of two isozymes, s and f. In this study, the biochemical properties of the s form partially purified from human placenta were compared with those of the f form from human liver. Only the placental s form has steroid sulfatase activity and hydrolyses estrone sulfate, dehydroepiandrosterone sulfate and cholesterol sulfate. The liver f form has barely detectable activity towards these sterol sulfates. With the artificial substrate, 4-methylumbelliferyl sulfate, both forms demonstrated a similar KM but the liver enzyme has a pH optimum of 6.9 while the placental form displayed two optima at 7.3 and 5.5. The molecular weight of the native enzyme determined with gel filtration was 183,000 for the s form and 200,000 for the f form and their pI's were also similar at 6.5. However, the T50, temperature at which half of the enzyme activity was lost, was 49.5 degrees C for the f form and 56.8 degrees C for the s form. Polyclonal antibodies raised against the placental form reacted specifically against the s and not the f form. They immuno-precipitated concomitantly greater than 80% of the total placental arylsulfatase C and steroid sulfatase activities while less than 20% of the liver enzyme was immuno-precipitable. In conclusion, the two isozymes s and f of arylsulfatase C in humans purified from placenta and liver, respectively, have similar KM, pI' and native molecular weight. However, they are distinct proteins with different substrate specificity, pH optima, heat-lability and antigenic properties. Only the s form is confirmed to be steroid sulfatase.