The proliferation of T lymphocytes in response to an antigenic stimulus requires the successive ligation of a range of T lymphocyte receptors by MHC-presented antigen, molecules expressed on the surfaces of accessory cells and lymphokines. A range of intracellular signalling systems are involved, with the possibility of alternative activation pathways utilising different intracellular signalling systems, though protein kinase C activation appears to play a key role. Appreciation of the complexity of the response may allow more selective clinical modulation of T lymphocyte-dependent immunological responses.