Unidirectional calcium influx can be measured by application of a filter paper blotter saturated with a 45Ca-labeled buffer solution to the mucosal surfaces of slit-open embryonic chick duodena which had been cultured mucosal side up on stainless-steel grids. The results obtained with this simple technique support the contention of a direct involvement of a vitamin D-induced calcium-binding protein in the transport of calcium across the mucosal surface. Permeability and inulin space measurements validate the method for measurement of calcium influx at initial velocity and obviate concern for "unstirred layer" effects. Kinetic analyses indicate a two step influx process, one saturable at about 5 mM calcium and the other essentially unsaturable.