The enzyme activity for α-naphthyl acetate esterase, naphthol-AS-acetate esterase, naphthol-AS-D-chloroacetate esterase, acid phosphatase, La(+)-tartrate-resistant acid phosphatase, adenosine triphosphatase, and 5’-nucleotidase was examined on the neoplastic cells of giant follicular lymphoblastoma, the so-called reticulum cell sarcoma and Sézary syndrome. The neoplastic cells of giant follicular lymphoblastoma showed distinct activity for adenosine triphosphatase and 5'-nucleotidase, and those of the so-called reticulum cell sarcoma had no characteristic nature of the reticulum cells or histiocytes enzyme histochemically. These findings suggest that these neoplastic cells may be derived from the B-cell system. In Sézary syndrome, acid phosphatase activity was localized in a small paranuclear area in Sézary cells, which were considered to have a T-cell nature. It is thought that these enzyme histochemical methods are easy and useful in differentiating the B- or T-cell nature and the classification of non-Hodgkin’s lymphomas.