In the present study, we have examined the kinetics and magnitude of expression of the CD28 and CD152 molecules on unstimulated and anti-CD3+rIL-2-stimulated peripheral blood CD4+ and CD8+ T cells in patients with chronic lymphocytic leukaemia (B-CLL) and controls. The mean percentages of both CD3+/CD4+/CD28+ and CD3+/CD8+/CD28+ cells were significantly lower in B-CLL than in controls before culture, decreased rapidly, reaching their lowest levels between 24 and 48 h, and returned to basal levels after 72 h of culture. In controls, the lowest proportions of CD3+/CD4+/CD28+ and CD3+/CD8+/CD28+ cells were found after 24 h and returned to prestimulation levels after 48 h of stimulation. We observed significantly higher proportions of unstimulated CD3+/CD4+/CD152+ and CD3+/CD8+/CD152+ cells in B-CLL patients than in controls. The highest percentages of CD3+/CD4+/CD152+ and CD3+/CD8+/CD152+ cells were observed in controls after 72 h, and in B-CLL patients after 24 h, and remained statistically higher after 48, 72 and 96 h of stimulation. CD152 molecule expression returned to prestimulation levels after 96 h of culture in controls, and after 120 h in B-CLL patients. The abnormal kinetics and levels of CD28 and CD152 expression on T cells in B-CLL may lead to a state of hyporesponsiveness or anergy and could be one of the mechanisms of immune deficiency in this disease.