Hairy-cell leukemia is characterized clinically in splenomegaly and pancytopenia and pathologically by the proliferation in hematopoietic tissue of cells containing the tartrate-resistant isozyme 5 of acid phosphatase. We have described a patient with a T-lymphocyte variant of this disease. A permanent cell line obtained from the spleen of this patient has the biological and enzymatic characteristics of the fresh leukemic cells. We have used this line to study the surface morphology, ultrastructure, and ultrastructural localization of acid phosphatase in defined T-lymphoid hairy cells. The surface of the cells of the permanent line was smooth but many hair-like projections appeared after exposure to phytohemagglutinin (PHA). There was little acid phosphatase reaction produce visualized when beta-glycerophosphate was used as a substrate. With sodium haphthol AS-BI phosphoric acid heavy deposits were seen in the perinuclear membrane, mitochondria, and rough endoplasmic reticulum. Exposure to PHA and pokeweed mitogen resulted in increased reaction product, suggesting increased enzyme synthesis. Tartrate-resistant acid phosphatase was localized in the same organelles.