The ultrastructural changes occurring when T cells specifically immune to antigens interact with P815 mastocytoma cells and EL4 lymphoma cells are described and related to changes previously observed by timelapse cinematography (Sanderson 1976b). In confirmation of work by others, pale T cells can clearly be incriminated as cytotoxic cells. Dark T cells also form contacts with target cells, and tend to project lamellipodia over the surface of the target cell. The possibility is discussed that these represent a subset of non-cytotoxic, antigen-reactive T cells involved in other T cell functions. T cells form two types of contact: relatively small point contacts, and large areas where the two plasma membranes are in close apposition. No structures resembling specialized junctions or membrane fusions were observed in areas of contact between T cells and target cells. Close contact between pale T cells and target cells is more regular than contact with dark T cells. Many contacts were seen between morphologically normal target cells and pale T cells, and these were thought to occur in the phase between contact and target cell death. Some of these pale T cells in contact show projections towards the centre of the target cell which invaginate the cell membrane, but do not penetrate it. Remarkable T cell projections were also seen which had penetrated deeply through the membrane of one target cell. These projections appeared to have disrupted the membrane and had penetrated into apparently intact cytoplasm, suggesting that this penetration may be an early event in the lytic mechanism. The possibility that this phenomenon is the cause of cell death is discussed. Changes corresponding to the phase of zeiosis of the target cell are described. These commence with the formation of surface blebs, accompanied by a general mis-shapening of the cell outline and followed by vacuolation and loss of cytoplasmic organelles. Breakdown of the nucleus appears to be a later, post mortem event.