Cells in many organs and tissues form interconnected systems. At the regions of cell contact, the membranes make junctions so structured as to form effective passageways between cells, where molecules, up to the order of 10,000 molecular weight can flow directly from one cell interior to the next. Cytoplasmic Ca plays a key role in the gating of the passageways particularly in tissue injury and healing. Certain cancerous cells (hepatoma and x-radiated) fail to make communicative junctions. The possibility is discussed of an involvement of the junction in growth control and differentiation.