IgG binds specifically to isolated jejunal enterocytes but not to ileal enterocytes; maximum binding occurred at pH 6. The ability of jejunal enterocytes to bind IgG was reduced to low levels at 20 days of age and was lost at 24 days. Human and rat IgG were bound specifically in similar amounts; human IgG displaced rat IgG with identical efficiency to rat IgG (ED50 = 50 nM). Much less bovine and sheep IgG were bound to enterocytes and the ED50s for these proteins were 150 nM and 2.5 microM, respectively. Rat IgG bound to jejunal enterocytes with high affinity (13.21 x 10(6)M-1) and to 4.83 x 10(6) sites per cell. Receptor protein was estimated to represent 0.18% of total cell protein. These observations are discussed in relation to the results of in vivo IgG transmission studies. It is estimated that the IgG transport mechanism, operating at maximum efficiency, requires that available IgG receptors would come into use once to twice per hour.