The aim of the present work was to quantify the passage of bovine immunoglobulins into the hemolymph of the tick Boophilus microplus during the feeding process and to determine their antibody activity. The knowledge is of paramount importance when vector control or blocking of disease transmission is attempted by vaccination of cattle. Approximately 2% of bovine immunoglobulin present in the serum as determined by competitive ELISA was demonstrated in hemolymph of B. microplus and antibody activity against an antigen of B. microplus in the hemolymph of ticks fed on bovine immunized with the antigen purified from tick eggs was detected by Western blot assay. The antibody reactivity detected against the B. microplus antigen showed that functional antibodies are present in the hemolymph of fully engorged ticks for at least 48 h after completing the parasitic life cycle.