Caco-2 cells were grown in bicameral chambers and ⁵⁹Fe transport into the basal chamber measured as described in Materials and Methods. The Fe transport determined for conditions in which the basal chamber contained varying concentrations either of apotransferrin, ovotransferrin, ferri-transferrin, cobalt-transferrin, DTPA, or dextran-DTPA. The rates were then fit with Michaelis-Menton kinetics and the K_m and V_max determined. K_m= sensitivity of effect. V_max = magnitude of effect at satutating concentrations. Shown are the mean ± SD for three experiments (with 6 to 8 points in triplicate for each experiment) except for the no addition (constitutive) rate of release, which is the mean of 12 experiments (n = 36). The values for K_m for apotransferrin were compared with those for the other basal chamber additions by a one-sided student t test. The V_maxs for all additions were compared by a one-sidedt test with the no addition rate of release. In addition the V_max for both DTPA and dextran-DTPA were significantly different from apo-Tf (P = .005 and P = .03, respectively).