A culture of Acer pseudoplatanus L. grown in the presence of an equilibrium level of 2,4-dichlorophenoxyacetic acid (2,4-D) of 1·5 × 10−7 M (state IV culture) showed, in comparison with one of a similar specific growth rate but in which the equilibrium level of 2,4-D was 2·3 × 10−6 M (state I culture), an enhanced degree of cell aggregation, enhanced mean cell volume, and the presence of cells giving a generalized lignin reaction with extracellular lignin-positive material. The state IV culture showed a proportion (10–15 per cent) of cells having ultrastructural features not observed in the state I culture. Some of the cells, located at the surface of the cellular aggregates, were small, rounded, highly cytoplasmic, and rich in rough endoplasmic reticulum. Further within the aggregates there occurred some cells showing abnormal or incomplete cytokinesis and having irregularly thickened walls. Located centrally in the aggregates were cells showing massive accumulations of electron-dense material and with cell walls showing bands of thickening alternating with thinner wall regions traversed by plasmodesmata. The latter cells are interpreted as cells showing intense polyphenol metabolism and imperfect xylogenic differentiation.