Binding of four blood group A-specific lectins, from Helix pomatia (HP), lucorum (HL), Cepaea nemoralis (CN) and Dolichos biflorus (DB) to human lymphocytes was determined. Use was made of lectins attached to latex particles, as convenient probes to detect binding of lectins to individual cells, Specificity of lectin-latex probes was demonstrated by comparison with agglutination and fluorescent lectin binding; experiments with untreated and enzyme-treated human and animal erythrocytes confirmed the specificity of the lectin-latex beads as did inhibition in the presence of N-acetyl-galactosamine, the immunodominant sugar of blood group A substance. The binding of HP to T cells and a proportion of B, and non-B, non-T cells is confirmed. Binding to B cells in chronic lymphocytic leukaemia (CLL) is also confirmed. HP-latex had the same specificity as soluble HP, but binding did not require pretreatment of lymphocytes with neuraminidase. HL-latex had the same specificity for erythrocytes, normal lymphocyte subpopulations and CLL lymphocytes as HP, but the binding was weaker and required pretreatment of the lymphocytes with neuraminidase to demonstrate it. Contrary to expectation, CN and DB bound to B lymphocytes and did not bind to T lymphocytes. CN binding was almost completely restricted to B cells, whereas DB bound to part of the non-B, non-T populations as well as to a substantial proportion of B cells. Neither CN nor DB labelled malignant T cells. Both CN and DB bound to B cells in CLL. As with HP, pretreatment of lymphocytes with neuraminidase increased the accessibility of receptors, but was not essential to demonstrate labelling with CN-, and DB-latex preparations. Binding of each lectin to lymphocytes was independent of ABO blood group of donor suggesting that ABO antigens may be expressed poorly, if at all, on lymphocytes. Binding is probably due to additional specificities possessed by these lectins. Protease treatment destroyed CN- and DB-binding sites but not HP receptors suggesting that at least two types of binding site are involved.