Previous reports were confirmed that specific binding sites exist on bovine mammary cells near parturition presumably involved in the transfer of immunoglobulins IgG1 and IgG2 across the mammary gland at the time of colostrum formation. Determination of the kinetic parameters of these binding sites using 125I-labeled IgG1 and IgG2 immunoglobulins indicated the presence of sites with association constants (Ka) of about 5 - 10(8)--10 - 10(8)M-1 for both subclasses during normal lactation with about 9000 and 3000 sites per cell for each, respectively. The number of IgG1 sites tended to increase as the time of parturition approached. In addition, a new group of sites numbering about 5000 per cell with very strong binding for IgG1 (Ka about 45 - 10(8)M-1) appeared on the cells about a week before parturition. The numbers and affinity of the IgG1 and IgG2 binding sites bear a relationship to the approximate 7 : 1 ratio of these immunglobulin subclasses found in colostrum and normal milk and to the time of maximum colostrum formation. The results support the premise that a highly selective transport mechanism exists in the bovine mammary epithelial cell for the transfer of IgG1 and IgG2 immunoglobulins from blood to the lacteal secretions.