Abstract: A method of manipulating the header field in an ATM cell. In accordance with the method, the ATM cell is transferred via an interface where each byte of the cell appears in a parallel form on a plurality of parallel data conductors and where information on the start of cell (SOC) is also available. In order to be able to carry out header bit manipulation in a simple manner at such an interface, data conductors corresponding to the bits to be manipulated are led to a circuit carrying out the manipulation, and the other data conductors are led past the circuit. The bit to be manipulated is searched for by counting from start of cell up to the byte containing the bit to be manipulated, after which the bit is manipulated. An output conductor corresponding to the manipulated bit is led from the manipulation circuit to its own position among the data conductors that bypassed the circuit.