Hairy cell leukaemia (HCL) cells on exposure to the tumour promoter tetradecanoyl-12,13-phorbol acetate (TPA) undergo a striking morphological change in culture, with the formation of long, slender cytoplasmic processes, and adhere to the plastic surface. By time lapse photography this change is seen to start within 10 min of exposure to the TPA and is characterised by slow but continuous extension and retraction of the processes. Apparent phagocytosis of sheep red cells but not of sensitised human red cells occurs. This response to TPA is prevented by cytochalasin D but not by 13 cis-retinoic acid. Lymphocytes from normal individuals and patients with chronic lymphocytic leukaemia and lymphoblasts from a patient with acute lymphoblastic leukaemia reacted by adhering to each other in small aggregates but did not adhere to the plastic surface. Monocytes adhere to plastic surfaces and respond to TPA by extension and retraction of cytoplasmic folds differing from the long slender processes formed by the HCL cells. Thus HCL cells are different in their response to TPA from both normal lymphocytes and monocytes.