A study was undertaken into the binding of 7 fluorescein (FITC)-labelled, lectins to sheep tissues and cells. Wheat germ agglutinin (WGA), conconavalin A (con A) and Ricinus communis agglutinin (RCA) bound strongly to all sheep tissues and circulating cells, while Dolichos biflorus (DBA) and Ulex europaeus (UEA) did not bind at all. Peanut agglutinin (PNA) and soybean agglutinin (SBA) bound to thymus and lymph node cell suspensions and to a proportion of circulating cells, but only weakly to the other sheep tissues tested. PNA and SBA bound to polymorphs and monocytes, making it necessary to treat whole blood with carbonyl-iron and magnet to remove the phagocytic cells before purfication of the peripheral blood leucocytes (PBL). PNA bound to 40 to 50% of carbonyl-iron treated PBL and to 65 to 70% of the cells in efferent popliteal lymph. Using a rhodamine-labelled anti-sheep lg reagent greater than 98% of PNA + cells in lymph and carbonyl-iron treated PBL were sIg-. Furthermore, using an FITC-labelled F(ab')2 anti-sheep Ig reagent, the sum of the 0/0 PNA+cells and sIg+ cells in individually labelled preparations was equal to the percentage of cells labelled when both reagents were added simultaneously. Cells binding PNA were found in more dense regions of buoyant density gradients and were less adherent to nylon wool than cells having surface immunoglobulin (sIg). Approximately 20 to 30% of PBL and 10 to 20% of the cells in lymph were PNA-/sIg- and these were termed 'null' cells. Although Con A induced sheep PBL to transform in tissue culture, PNA, SBA and WGA all failed to stimulate the incorporation of 3H-thymidine at the concentrations tested. Sorting lymph cells from BCG vaccinated sheep into PNA+ and PNA- subpopulations showed that the cells able to transform in vitro to the specific antigen, purified protein derivative (PPD) or the mitogen phytohemagglutinin (PHA) were in the PNA+ but not PNA- population.