The helper and suppressor capacity of T, T mu (T non gamma) and T gamma cells was assessed in a group of patients with B-cell chronic lymphocytic leukaemia (B-CLL) in a pokeweed mitogen (PWM) stimulated system. The enriched T-cells (E-rosette positive) from all B-CLL cases showed a reduced capacity to induce the differentiation of normal B-lymphocytes compared with normal T-cells (P less than 0.005). After enrichment of the T mu cells, the helper/inducer capacity was still significantly depressed compared with the same fraction from normal controls (P less than 0.01). On the other hand, enriched T gamma cells from B-CLL were effective in suppressing the differentiation of normal B-lymphocytes to a similar degree as normal T gamma cells. These findings are indicative of a deficient T-cell helper function in B-CLL, which appears to be unrelated to the clinical stage of the disease. The fractionation experiments suggest that this functional impairment is not only due to the abnormal T-cell subset distribution seen in the majority of cases, but point to a possible intrinsic defect within the T mu cell population.