It may be useful, at least for now, to visualize the signal to stop growth in cell "contact inhibition" as electrical rather than chemical. Chains of cells, which have been traced in tissue as low-resistance pathways, can be regarded as transmission lines with high-pass filter characteristics. Such lines could communicate cell contact signals deep into tissue. No such signals would be received from (or by) cancerous tisdue cells on contact at one end of the line because such cells fail to form the low-resistance, low-capacitance junctions now believed to be formed by all normal cells in connective tissue. © 1969.