Unfractionated normal blood lymphocytes (predominantly T lymphocytes) showed significantly elevated total and neuraminidase-susceptible sialic acid compared with CLL lymphocytes (predominantly B lymphocytes). T-enriched fractions from normal blood contained about three-fold the sialic acid content of the corresponding T-depleted fractions, and similarly low values were also obtained for T-depleted fractions from tonsils. Thus the reduced sialic acid levels in CLL lymphocytes appear to be due to their B cell nature. In contrast to the situation in normal peripheral venous blood, tonsillar T and B cells did not differ markedly in sialic acid content, suggesting that for T cells maturity and/or location in vivo are important determinants of this parameters. Results with calf thymocytes were in agreement with this. Sialic acid was estimated in malignant B and T lymphoblasts. Shedding of sialic acid-containing moieties from lymphocytes was also examined.