Transport study of horseradish peroxidase and transferrin-horseradish peroxidase conjugate was performed using an in vitro Caco-2 cell cultured monolayer grown on a polycarbonate membrane of Transwell^ⓡ. Horseradish peroxidase was not transported across Caco-2 cell monolayer. Transferrin-horseradish peroxidase conjugate was transported through Caco-2 cell monolayer. The apparent membrane permeability coefficient (P_(app)) of transferrin horseradish peroxidase conjugate was 6.54×10^(-7) ㎝/sec. The P_(app) value of transferrin-horseradish peroxidase conjugate across Caco-2 cell monolayer was increased to 11.9×10^(-7) ㎝/sec in the presence of 50 ㎍/㎖ brefeldin-A. These results suggest the transferrin receptor mediated transcytosis of transferrin-horseradish peroxidase conjugate across Caco-2 cell monolayer.