size limit of molecules permeating the junctional membrane channels the permeability of the celltocell membrane channels in salivary gland cell junction chironomus thummi was probed with fluorescentlabeled amino acids and synthetic or natural peptides molecules up to 1200 daltons pass through the channels with velocities depending on molecular size molecules of 1900 daltons or greater do not pass this passage failure seems to reflect the normal size limit for junctional channel permeation the channels continue to be permeated by the molecules up to 1200 daltons when these are mixed with the nonpermeant molecules from this size limit a channel diameter of 10 to 14 angstroms is estimated