The interaction of phytohaemagglutinin (PHA) with the human T lymphocyte antigen receptor (Ti) was explored. Nonidet‐P40 lysates of surface‐labelled HPB‐ALL cells were immunoprecipitated with PHA, using a rabbit anti‐(PHA)‐serum, as well as clonotypic monoclonal antibodies (H1‐2D4 and T40/25) and a rabbit antiserum (R‐43) against Ti. One‐ and two‐dimensional SDS‐polyacrylamide electrophoresis under reducing and non‐reducing conditions showed that both the clonotypic antibodies and PHA precipitated a disulphide cross‐linked heterodimer having a mol. wt. of approximately 79 000 (unreduced) and a comprising subunits of mol. wts. approximately 50 000 and 39 000 (reduced). Further evidence that PHA binds Ti was obtained by (i) cross‐immunodepletion with H1‐2D4 and PHA; (ii) immunoprecipitation with H1‐2D4 of a glycoprotein fraction specifically eluted from a PHA immunoprecipitate; (iii) immunoprecipitation with PHA of a solubilised H1‐2D4 immunoprecipitate; (iv) 2‐D (non‐equilibrium pH gradient electrophoresis/SDS) analyses of H1‐2D4 and PHA immunoprecipitates, indicated that H1‐2D4 and PHA recognise coincident beta polypeptides. PHA also binds a Ti‐like disulphide cross‐linked heterodimer on tonsil lymphocytes and two other T‐cell leukaemias (HUT‐78 and J6). The data further suggest that PHA and R‐43 recognise a subpopulation of Ti molecules on HPB‐ALL cells that are not bound by H1‐2D4, suggesting that there may be at least two forms of Ti. Similar experiments indicate that Concanavalin A (Con A) and wheat germ agglutinin (WGA) also probably bind Ti, whereas Helix pomatia agglutinin (HPA) does not.