In order to determine the relationships between CD2+ lymphocyte subpopulations and tumour mass, the immunophenotype of natural killer (NK) cells and T lymphocyte subsets was studied in 56 B-chronic lymphocytic leukaemia (B-CLL) patients and 38 healthy subjects. The patients were classified according to their blood lymphocyte count (BLC). Forty patients had BLC<30x10(9)/l (low BLC, less tumour mass) and 16 patients had BLC>30x10(9)/l (high BLC, larger tumour mass). The percentage of CD3- CD56+ cells, as well as of CD8+, CD8+ CD45RO+ and CD3+ CD57+ T subsets in low BLC patients, were higher than those found in high BLC patients. Conversely, the percentages of CD3+ HLA x DR+, CD4+ and CD4+ CD45RO+ lymphocytes were higher in high BLC patients than in low BLC patients. The CD4/CD8 ratio was decreased in low BLC patients while it was increased in high BLC patients and a significant positive correlation was found between their CD4/CD8 ratio and their BLC. We conclude that in low BLC B-CLL patients there is a decreased percentage of activated helper lymphocytes and an increased percentage of NK cells and activated cytotoxic T lymphocytes. These results suggest a role for NK cells, and helper and cytotoxic T lymphocytes in the control of tumour burden in B-CLL patients.