The cell-to-cell channels in the junctions of an insect salivary gland and of insect and mammalian cells in culture were probed with fluorescent molecules-neutral linear oligosaccharides, neutral branched glycopeptides, and charged linear peptides. From the molecular dimensions of the largest permeants and smallest impermeants the permeation-limiting channel diameter was obtained: 16 to 20 angstroms for the mammalian cells and 20 to 30 angstroms for the insect cells.