Kurloff cells are mononuclear cells characterized by a large metachromatic and PAS-positive inclusion called the Kurloff body. Bone-marrow and spleen Kurloff cells were incubated with p-nitrocatechol sulfate as substrate and barium chloride as capturing agent for the ultracytochemical detection of the lysosomal marker enzyme, arylsulfatase. Enzymatic reaction product was consistently found as a single spot-like deposit confined to the rim of the Kurloff body. These results, and the previously described presence of other acid hydrolases and sulfated glycosamino++glycans, emphasize the similarities between the Kurloff body and lysosomes. Reaction product could also be found occasionally in segments of the rough endoplasmic reticulum but it was absent from the Golgi apparatus. This arylsulfatase activity could be related to the natural killer activity of Kurloff cells.