The behaviour of 5'-nucleotidase isoenzymes (ecto-5'-nucleotidase, e-Ns and c-N-II soluble 5'-nucleotidases) was studied in lymphocytes from patients with B-cell chronic lymphocytic leukemia. A strong reduction in ecto- and soluble activities was observed, although the pattern of the three 5'-nucleotidases did not always strictly overlap. A significant decrease (p<0.05) in ecto-5'-nucleotidase, e-Ns and c-N-II was found in B and T populations (B lymphocytes: 1.13, 0.88 and 1.26 nmol/h/10(6) cells versus 95.96, 9.64 and 13.73 nmol/h/10(6) cells in controls; T lymphocytes: 1.31, 0.23 and 0.06 nmol/h/10(6) cells versus 9.25, 1.31 and 2.10 nmol/h/10(6) cells in healthy subjects). The percentage of ecto-5'-nucleotidase-positive cells (CD73+) was reduced in leukemia patients, indicating a lower number of active molecules on the cell surface. The results of RT-PCR analysis showed that the ecto-5'-nucleotidase mRNA of leukemia patients was not defective.