The ultrastructural localization of four acid hydrolases (acid phosphatase, beta-glucuronidase, beta-glucosaminidase and alpha-naphthylacetate esterase) has been studied in lymphocytes from 16 patients with three types of chronic T-cell leukaemia, namely, T-prolymphocytic leukaemia (T-PLL), T-chronic lymphocytic leukaemia (T-CLL) and adult T-cell lymphoma leukaemia (ATLL). Different patterns of enzyme distribution were observed in the leukaemic T-cells from these disorders. In T-PLL, reactivity for the four acid hydrolases was confined to single or a few large granules. Gall bodies were reactive for beta-glucuronidase, b-glucosaminidase and alpha-naphthylacetate esterase but apparently unreactive for acid phosphatase. In T-CLL, scattered small- to medium-size cytoplasmic granules and many parallel tubular arrays were strongly reactive for acid phosphatase, beta-glucuronidase and beta-glucosidase but showed no reactivity for alpha-naphthylacetate esterase. Intermediate features were observed in ATLL. The observed differences in enzyme reactivity reflect a different content of lysosomal granules in the various types of leukaemic T-cells. They also suggest that similar differences may be found in normal T-lymphocyte subsets.